Compliant blood vessel graft

ABSTRACT

Stents and methods of using stents are provided. Stents of the invention provide external support structure for a blood vessel segment disposed within, wherein the stents are capable of resilient radial expansion in a manner mimicking the compliance properties of an artery. The stent may be formed of a knitted or braided mesh formed so as to provide the needed compliance properties. A venous graft with the stent and a vein segment disposed within is provided, wherein graft is capable of mimicking the compliance properties of an artery. Methods of selecting stents for downsizing and methods of using the stents of the invention in downsizing and smoothening are provided. Methods of replacing a section of an artery with a venous graft including a stent of the invention are provided. Methods of reducing intimal hyperplasia in implanted vein segment in a venous graft using stents of the invention are provided.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation-in-part of U.S. patentapplication Ser. No. 10/987,313, filed on Nov. 12, 2004 and currentlypending, which is a continuation-in-part of U.S. patent application Ser.No. 10/834,360, filed on Apr. 28, 2004 and currently pending, whichclaims the benefit of priority from U.S. Provisional Patent ApplicationSer. No. 60/466,226, filed on Apr. 28, 2003, the contents of which areall incorporated herein by reference in their entireties.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is generally related to a graft involving a bloodvessel segment and a supportive sheath chosen to provide the graft withmechanical compliance properties which resemble those of a healthynative artery, and a method for sizing such a graft.

2. Description of Related Art

Various types of vascular prostheses are known or available.Commercially available synthetic vascular grafts in use are commonlymade from expanded polytetrafluoroethylene (e-PTFE), or woven, knitted,or velour design polyethylene terephthalate (PET) or Dacron®. Theseprosthetic vascular grafts may have various drawbacks. When used forrepairing or replacing smaller diameter arteries, these grafts may faildue to occlusion by thrombosis or kinking, or due to an anastomotic orneointimal hyperplasia (exuberant cell growth at the interface betweenartery and graft). Another problem may involve expansion and contractionmismatches between the host artery and the synthetic vascularprosthesis, which may result in anastomotic rupture, stimulatedexuberant cell responses, and disturbed flow patterns and increasedstresses leading to graft failure.

Problems also exist with the use of autologous saphenous vein grafts inthese applications. Use of autologous saphenous vein grafts to bypassblockages in coronary arteries has become a well-established procedure.However, their success in the long term has been limited. In thecoronary position, the literature reports a low (45-63%) patency of veingrafts after 10-12 years. It is believed that these failures result fromremodeling of the implanted vein in response to greatly increasedinternal pressure, that is, as the vein is required to function as anartery. In general, arteries have substantial musculature and, althoughable to expand diametrically in response to increased internal pressure,are capable of withstanding normal arterial pressure variances. Veins,on the other hand, are not required to withstand arterial pressurevariances and are relatively incapable of withstanding the higherarterial pressures without substantial bulging. In this regard, thenominal venous diameter seen under nominal venous pressure is seen toapproximately double upon exposure to arterial pressure.

Increases in lumenal diameter of these magnitudes in vein segmentimplants are accompanied by increases in tangential stress. Tangentialstress has been shown to be proportional to the lumenal radius-wallthickness ratio. In healthy arteries, this ratio remains constant acrossmultiple species. However, this does not occur in veins. It is believedthat a vein's smooth muscle cells increase their growth rate and secreteextra-cellular matrix components in response to such increases intangential stress. This becomes a remodeling response, and is likely anattempt by the vein to reduce the lumenal radius-wall thickness ratio,and consequently the tangential stress. However, it appears that thesereactions overcompensate in the veins, resulting in the phenomenon ofneointimal hyperplasia yielding grossly thickened and stiff graft walls.As the dilation of the vein segment continues, the resulting mismatchbetween the vein and artery diameters may lead to disturbance of flowpatterns, which may also favor the formation of thrombi.

Problems also exist when tubular prostheses are used as exteriorlyaccessible shunts to facilitate access to the circulatory system for,e.g., the administration of medicines and nourishment and for dialysisprocedures.

For several decades saphenous vein grafts have been the most widely usedarterial bypass conduits. As much as there is an increasing trendtowards the use of arterial grafts such as the internal thoracic-,radial- or gastroepiploic artery, the saphenous vein will remain anindispensable conduit for large numbers of patients. This isparticularly true for lower limb reconstructions where artery grafts arenot available.

Although the overall patency of saphenous vein grafts is distinctlybetter than that of synthetic conduits, the failure rate of vein graftsis still sobering when compared with artery grafts. The main reason forthe failure of vein grafts is the development of intimal hyperplasia.Since late vein graft failure due to arteriosclerotic degeneration alsodevelops on the bed of intimal hyperplasia, this subintimal tissuedevelopment holds the master-key to poor vein graft performance. Theconsequences of this shortcoming are dramatically illustrated by thefact that one third of all peripheral vascular operations are revisionsand at 5 years 50% of all peripheral grafts needing revision for failureled to an amputation.

It is well recognized that there are two major forms of intimalhyperplasia: a diffuse and a focal one. While diffuse intimalhyperplasia often regresses, focal intimal hyperplasia tends toprogress, leading to a significantly higher occlusion rate. The overalltriggers for both forms of intimal hyperplasia are low shear stress atthe blood interface and high circumferential wall stress—both related tothe significantly larger cross sectional area of the vein graft than thetarget artery and exposure to arterial pressure. The aggravating factorsin focal narrowings, however, are areas of particularly low fluid shearstress and increased shear gradients. Eddy flow as a consequence ofuneven lumenal dimensions was shown to be the reason behind thesehaemodynamic conditions causing focal intima hyperplasia. Independently,wall irregularities were shown to be the main predisposing condition forfocal intimal hyperplasia.

As early as in the 1960s attempts were made to restrict the expansion ofvein grafts in the arterial circulation and eliminate uneven lumenaldimensions through external mesh-support with diameter reduction. Sincethen, many investigators have researched this field but the translationinto clinical practice was limited to last-resort measures in varicoseveins.

BRIEF SUMMARY OF THE INVENTION

It has now been found that a blood vessel segment such as a veinsegment, if externally supported by an appropriate, flexible,radially-resiliently tubular support, can provide a valuable tubularprosthesis. A vein segment so supported can function in much the samefashion as the artery that is to be replaced. That is, it functionswithout undue bulging or aggravated mismatching phenomena leading tograft failure. Unless otherwise indicated, the term “compliance” meansthe ratio of the diameter change of a vessel as it expands in the radialdirection in response to a given change in vessel pressure, and thevalues for compliance referred to below result from dynamic, in vitrotesting. The terms “venous graft” and “vein graft” are usedinterchangeably herein. As described in greater detail below, thecompliance of venous graft (vein graft) is largely dependent upon thecompliance of the external, radially resilient support.

The invention in one embodiment, accordingly, relates to a flexible,resilient, generally tubular external support within which may besupported a blood vessel segment such as a vein segment to form a graft.The tubular support is capable of resilient radial expansion in a mannermimicking the compliance properties of an artery, and compliance figuresin the range of 3 to 30%/100 mm Hg are appropriate. The tubular supportmay be formed of a knitted or woven mesh that is so formed as to exhibitthe needed compliance properties.

The invention in certain embodiments provides a venous graft (veingraft) for replacement of a section of an artery. The graft comprises aflexible, resilient, generally tubular external support and a veinsegment carried within and having an ablumenal surface in contact withand supported by the tubular support, the venous graft being capable ofresilient radial expansion in a manner mimicking the complianceproperties of an artery. Compliance figures in the range of 3 to 30%/100mm Hg are appropriate, although compliance values ranging up to 50%/100mm Hg may be desired in some instances. The tubular support may take theform of a fiber mesh, such as a knitted, braided or woven mesh, thefibers of which may, if desired, be appropriately crimped to provide therequired resiliency and compliance. The fiber mesh may be made of analloy or a polymer material as further described in the application.

The invention in certain embodiments provides a venous graft (veingraft) for replacement of a section of an artery, where the graftcomprises a flexible, resilient, generally tubular external supporthaving a loosely knitted (“loose-knit”) mesh structure, and a veinsegment carried within and having an ablumenal surface in contact withand supported by the tubular support, the venous graft being capable ofresilient radial expansion in a manner mimicking the complianceproperties of an artery. The tubular support having a loosely knittedmesh structure having the required resiliency and compliance may furtherprovide smoothening of irregularities , e.g., by reducing or eliminatingdifferences between the outer diameter of a section of the stented veinsegment and adjacent vein sections to provide a vein with an outerdiameter substantially the same along its length. In certainembodiments, the tubular support having a loosely knitted mesh structuremay exhibit limited shrinkage after graft implantation, providingfurther smoothening by post-implantation downsizing of the graftdiameter. The loosely knitted mesh for use in the tubular support may bemade of an alloy or a polymer material as further described in theapplication.

In other embodiments, the invention relates to a method for producing avenous graft (vein graft) for use, for example, in replacing a sectionof an artery. A segment of a vessel is provided, and is sheathed in agenerally tubular support in supportive contact with the ablumenalsurface of the vein segment. The support is sufficiently flexible andradially resilient as to provide the resulting graft with complianceproperties mimicking the compliance properties of the artery to bereplaced. Sheathing of the vessel segment within the tubular support maybe accomplished by supporting the generally tubular support upon anexterior surface of an applicator having an internal passage withinwhich is positioned the vessel segment, and removing the applicator topermit the tubular support to come into supportive contact with theablumenal surface of the vessel segment. Axial dimensional changes inthe tubular support may be controlled as necessary to provide the graftwith the desired compliance properties mimicking arterial complianceproperties. The tubular support may take the form of a fiber mesh asdescribed herein, made of an alloy or a polymer material, chosen tooptimize the compliance properties of the graft so that the stentedgraft is evenly compliant across variations in structure of theharvested vein segment.

Other embodiments of the invention relate to vessel grafts that includea flexible, resilient, generally tubular external support formed of ashape memory alloy, and a vessel segment carried within and having anablumenal surface in contact with and supported by the tubular support.The shape memory support may be placed around a vessel segment when theshape memory material is in a first enlarged configuration. The tubularsupport comes into supportive contact with the ablumenal surface of thevessel when the support is transformed, as by a temperature increase orupon removal of an introducer tube over which the tubular support issupported, into a second configuration different from the firstconfiguration. The shape memory support in its second configuration mayexhibit superelastic properties and in any event is sufficientlyflexible and resilient as to provide the venous graft with complianceproperties mimicking the compliance properties of, for example, anartery. Compliance figures in the range of 3 to 30%/100 mm Hg areappropriate. The tubular support may take the form of a wire mesh madeof shape memory alloy, such as a knitted or woven mesh, the wires ofwhich may, if desired, be appropriately crimped to provide the requiredresiliency and compliance.

The invention is described hereafter primarily with respect to graftsthat utilize veins that are received within a tubular support and thatcan function as replacements for arterial segments in, for example,coronary by-pass procedures, but the grafts of the invention may alsoutilize other vessels such as arteries, including treated vein andartery segments from donor animals such as vessels of porcine and bovineorigin.

In certain embodiments, the invention relates to a method for selectinga stent for a venous graft, by measuring a minimum diameter and amaximum diameter of a vein, selecting a maximum amount of downsizing forthe vein and a minimum amount of downsizing for the vein, calculating arange of diameters of stents that provide the an amount of downsizingbetween the selected maximum amount and the selected minimum amount, andselecting a single stent having a diameter that falls within thecalculated range. The method can further include calculating a degree ofdownsizing for smoothening the vein, by altering an outside diameter ofat least one part of the vein to be substantially the same as anotherpart of the vein. In certain embodiments, the single stent selected bythis method is the smallest possible stent within the range, resultingin maximum downsizing. In other embodiments, the single stent selectedby this method is the largest possible stent within the range, resultingin minimum downsizing. In certain embodiments, the single stent selectedby this method has a diameter of between about 2.7 mm and about 4.0 mm,more particularly between about 3.0 mm and 4.0 mm. In accordance with anaspect of the invention, the maximum amount of downsizing for the veinis the degree of downsizing for substantially smoothening the vein,where substantially smoothening the vein can include smootheningirregularities in the vein.

The invention in certain embodiments provides a stent including agenerally tubular member constructed and arranged to receive a harvestedvein segment, the generally tubular member being compliant so as tocontract and expand with the vein, the tubular member having an innerdiameter between about 2.7 mm and about 4.0 mm, or between about 3.0 mmand about 4.0 mm. In various non-limiting embodiments, the generallytubular member can be a knitted structure, where the knitted structurecan be a metal wire, or a polymeric material, in particular anelastomeric polymer. In accordance with one aspect of the invention, theknitted structure can be configured to be shrinkable after receiving theharvested vein segment. In certain embodiments, the stent can include aplurality of connected rings, optionally interconnected rings,optionally wherein the rings are connected on outside surfaces of therings.

In another embodiment, the invention provides a stent delivery deviceincluding a stent constructed and arranged to receive a vein segment,where the stent is compliant so as to contract and expand with the veinsegment, and a delivery tube comprising a coating configured to impartslip properties to the tube to reduce traumatic introduction of thestent to an outside surface of the vein.

In another embodiment, the invention relates to a method of stabilizinga vein segment in a venous graft, the method by providing the veinsegment, disposing the vein segment within a lumen of a stentconstructed and arranged to contract and expand with the vein segment,where the stent has an inner diameter between about 2.7 mm and about 4.0mm. In various non-limiting embodiments, the stent can have a knittedstructure, where the knitted structure can be made of metal wire, or apolymeric material. In accordance with one aspect of the invention, theknitted structure can be configured to be shrinkable after receiving theharvested vein segment.

In one embodiment, the invention relates to a method for replacing asection of an artery in a patient with a venous graft capable ofresilient radial expansion in a manner mimicking the complianceproperties of a healthy artery in a patient. In accordance with thisaspect, steps of the method can include, but are not limited to,restricting blood flow through the section of artery to be replaced,excising the section of artery to be replaced, leaving a first availableartery end and a second available artery end in the patient, providing avein segment having a first vein segment end and a second vein segmentend, joining the first vein segment end to the first available arteryend, providing a flexible, resilient, generally tubular external supportstent capable of resilient radial expansion in a manner providingcompliance in the range of 3 to 30%/100 mm Hg, sheathing the veinsegment with the stent by introducing the second vein segment end intothe stent and sliding the stent over the ablumenal surface of said veinsegment until substantially all of said vein segment is carried withinsaid stent; and joining the second vein segment end to the secondavailable artery end provide said venous graft, wherein said venousgraft is capable of resilient radial expansion in a manner mimicking thecompliance properties of a healthy artery when blood flow is restored tosaid artery. In accordance with this aspect, the stent can be capable ofsaid resilient radial expansion without significant axial dimensionalchanges. In accordance with another aspect, the stent can include agenerally tubular fiber mesh capable of expanding in diameter throughresilient movement of fibers of the mesh to accommodate radial expansionof the vein segment supported in it sufficient to provide the venousgraft with the compliance. In certain embodiments, the stent includes aknit, tubular mesh capable of expanding radially to accommodate radialexpansion of the vein segment supported in it, within said compliancerange. In certain embodiments, the stent comprises a braided fiber meshso configured as to exhibit radial expansion in said compliance rangewithout significant reduction in the axial length of the stent. Incertain embodiments, the fiber mesh is made of metal wire, optionally ashape memory alloy. In other embodiments, the fiber mesh is polymeric.In some embodiments, the ablumenal surface of the vein segment is bondedto said stent.

In one embodiment, the invention relates to a method for reducingintimal hyperplasia in an implanted vein segment following replacementof a section of an artery with a implantation of a venous graftcomprising a providing venous graft comprising a flexible, resilient,generally tubular external support stent and said vein segment carriedwithin the stent, where the vein segment has an ablumenal surface incontact with and supported by the stent, wherein the venous graft iscapable of resilient radial expansion in a manner mimicking thecompliance properties of a healthy artery when blood flows through thevenous graft under physiological conditions. In certain embodiments, thestent support is capable of resilient radial expansion in a mannerproviding compliance in the range of 3 to 30%/100 mm Hg, and maybecapable of said resilient radial expansion without significant axialdimensional changes. In accordance with one aspect, the stent caninclude a generally tubular fiber mesh capable of expanding in diameterthrough resilient movement of fibers of the mesh to accommodate radialexpansion of the vein segment supported in it sufficient to provide thevenous graft with the compliance range, optionally a knit, tubular meshor a braided fiber mesh.

In certain embodiments, the invention relates to a method for reducingintimal hyperplasia in an implanted vein segment by providing a venousgraft with a knitted fiber mesh wherein the lumen diameter of saidimplanted vein segment does not increase significantly over time, i.e.,remains substantially isodiameteric. In other embodiments, the methodfurther includes shrinking the stent after disposing the vein segmentwithin the lumen of the stent to smoothen irregularities in the veinsegment. In other embodiments, the method further includes selecting astent for the venous graft by measuring a minimum diameter and a maximumdiameter of a vein from which the vein segment will harvested, selectinga maximum amount of downsizing for said vein and a minimum amountdownsizing for the vein, calculating a range of diameters of stents thatprovide the amount of downsizing between the selected maximum amount andthe selected minimum amount, and selecting a single stent having adiameter that falls within the calculated range.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a pressure versus diameter graph typifying the characteristicsof a native vein, native artery, a non-compliant stented vein, and acompliant stented vein;

FIG. 2 is a schematic cross-sectional view of an artery;

FIG. 3 is a representative pressure versus strain graph;

FIG. 4 is a pressure versus graft diameter graph;

FIG. 5 is a photograph of a tubular support in a first configuration,shown in an axially compressed and radially expanded configuration andsupported on a plastic tube;

FIG. 6 is a photograph of the tubular support of FIG. 5 in an axiallyelongated and radially reduced configuration to conform to a vein outerdiameter;

FIG. 7 is a side view of the graft of FIG. 6, showing a length-governingelement;

FIG. 8 is a schematic view of braided elements;

FIG. 9 is a perspective view of a braided tubular support;

FIG. 10 is a schematic view of knitted elements;

FIG. 11 is a side view of a section of a knitted tubular support;

FIG. 12 is a view of angular pre-braiding crimped elements;

FIG. 13 is a perspective, schematic view of an angular pre-braidingcrimped tubular support;

FIG. 14 is a view of rounded pre-braiding crimped elements;

FIG. 15 is a view of angular pre-knitting crimped elements;

FIG. 16 is a view of rounded pre-knitting crimped elements;

FIG. 17 is a broken-away, perspective view of a post-braiding crimpedtubular support;

FIG. 18 is a broken-away, perspective view of a venous graft showing aportion with anti-fraying element;

FIG. 19 is a broken-away, perspective view of one embodiment utilizingan applicator for assembling a venous graft;

FIG. 20 is a broken-away, perspective view of the use of a modifiedapplicator for assembling a venous graft;

FIG. 21 is a perspective view of a section of a knit tubular support;

FIG. 22A is a schematic cross-sectional view of an assembly device;

FIG. 22B is a schematic, prospective view of a step in the assembly of avessel graft;

FIG. 23 is a schematic, prospective view of another step in the assemblyof a vessel graft;

FIG. 24 is a schematic cross-section of an attachment of a vessel to atubular support;

FIG. 25 is a schematic cross-section of another attachment of a vesselto a tubular support;

FIG. 26 is a schematic cross-section of yet another attachment of avessel to a tubular support;

FIG. 27 is a schematic cross-section of the attachment of a vessel to atubular support utilizing a sleeve;

FIG. 28 is a schematic cross-section of the attachment of a vessel to atubular support utilizing an adhesive tape;

FIG. 29A is a cross section of a clip bearing an adhesive tape segment;

FIGS. 29B through D are schematic views showing stages in theapplication of an adhesive tape segment to a vessel graft;

FIG. 30 is a schematic view showing severance on a bias of the vesselgraft also shown in FIG. 29D;

FIG. 31 is a schematic view of the attachment to an artery of a segmentshown in FIG. 30;

FIG. 32 is a photograph of a portion of a bioprosthetic access vesselgraft;

FIG. 33 is a side view of a section of another embodiment of a knittedtubular support;

FIG. 34 is a side view of a section of another embodiment of a knittedtubular support;

FIG. 35 is a side view of a section of another embodiment of a knittedtubular support;

FIG. 36 is a side view of a section of another embodiment of a knittedtubular support;

FIG. 37 is a schematic view of a section of a knitted tubular supportshowing various dimensions of the support;

FIG. 38 is another schematic view of the knitted tubular support of FIG.37 showing other dimensions of the support;

FIG. 39 is a table listing the dimensions shown in FIGS. 37 and 38 ofthe knitted tubular supports of FIGS. 33-36;

FIG. 40 is a schematic view of another embodiment of a tubular support;

FIG. 41 is a schematic view of another embodiment of the tubular supportof FIG. 40;

FIG. 42 is a schematic view of another embodiment of a tubular support;

FIG. 43 is a schematic view of another embodiment of the tubular supportof FIG. 42;

FIG. 44 is a flow chart of a method to select a stent for smoothening ahuman saphenous vein;

FIG. 45 is a flow chart of another method to select a stent forsmoothening a human saphenous vein;

FIG. 46 is a graph showing individual admissible stent diameter rangesfor a set of veins from an experimental study;

FIG. 47 is a graph showing maximum to minimum outer diameterrelationships of saphenous veins distended during leak testing duringclinical vein harvest for coronary artery bypass surgery; and

FIG. 48 is hypothetical cumulative distribution plot showing increasingprobability of successful results from the theoretical study.

DETAILED DESCRIPTION OF THE INVENTION

Applicants have recognized that significant deficiencies attend to thepast methodologies and devices relating to the increased pressuresexperienced by vein grafts (venous grafts) utilized in arterialpositions. The increased pressures lead to excessive dilation of veingrafts in arterial circulation, leading to the development of intimalhyperplasia, which causes occlusion of the vessel.

Intimal hyperplasia is believed to be a primary reason for vein graftfailure. In this context it is known that intact endothelium acts in amanner to protect against the proliferation of underlying vascularsmooth muscle cells, known as VSMC. The intact endothelium also plays arole in VSMC contractile responses. The VSMC have also been shown torelease factors with long term physiological effects on the endothelialcells, including maintenance of a non-proliferative state. Bycomparison, the pathogenesis of intimal hyperplasia in a vein graft mayfollow the sequence of dilatation under arterial pressure;overstretching to maximum capacity; disruption of borders of endothelialcells; rupture of internal elastic membranes; migration of smooth musclecells into the intimal layer and resultant unbalanced proliferation;atrophy of media and further consolidation of stiffness; and graftarteriosclerosis with traumatic media necrosis and atrophy, as well aspathological surface and wall stress and strain. These phenomena mayresult in a decrease in vein graft patency within six years. Initialhyperplasia may be observed in such grafts from about 16 months, whileanastomotic intimal hyperplasia may occur at about 18 months, andarteriosclerosis may occur from about 45 months.

Others have attempted to overcome certain of these problems by use ofmetallic or polymeric external structures designed to arrest thedilation of the vein graft. FIG. 1 graphs blood pressure against vesseldiameter, with D_(O) representing the vessel diameter at zero pressure.As shown in this graph, lines 16, 18 represent the normal diastolic,i.e. low (80 mm Hg) and normal systolic, i.e. high (120 mm Hg)physiological blood pressure range for humans. Line 21 may represent thediameter of an artery (D_(A)) at 100 mmHg, and line 23 may represent thediameter of a vein (D_(V)) at the same pressure of 100 mmHg. Anunstented native artery reacts to pressure loading as shown at line 32,and an unstented vein reacts to the same loading as shown at line 35.The use of known stents with vein grafts results in movement of line 35in the direction shown by arrow 38, resulting in the approximate profileindicated at line 42 showing the response of a pressure loaded vein andnon-compliant stent combination. Although this prevents over-dilation,and some advantage accrues, this may lead to further unhealthy sequelae.Also, to the extent that vein-stent combination devices may be shown tolimit some of the dilation and intimal hyperplasia in the mid-graftregion, they may not be able to prevent intimal hyperplasia at theanastomoses. This can be a significant problem for vein grafts that aretransplanted into the arterial circulation vasculature. Prior attemptsto resolve these problems fail to recognize the full implications of avein being used in these situations. Accordingly, factors in the designof a vein-graft that may have a significant impact on its long termpatency may have been missed.

One important factor in proper remodeling is that of proper cyclicstretch. Applicants are able to incorporate this concept into vein-stentgrafts of the invention. In similar manner, the role of vascularendothelial growth factor (VEGF) in vascular smooth muscle cells may bevery important to the design of a preferred arterial vein-stent graft.It is known that low concentrations of VEGF may play a role inpreserving and repairing the arterial lumenal endothelial layer.Further, it is suggested that activation of the VEGF receptor KDR isaffected by cyclic stretch. Applicants believe that the phenomenon ofupregulation of VEGF expression by physiological stretching of vascularsmooth muscle cells is one reason for redesigning a vein-stent graftwhich has improved, controllable cyclic stretch features.

A further consideration is the influence of tensile stress/strain on thestructure and organization of smooth muscle cells during development andremodeling, particularly as to the orientation of such cells. In alarger topographical sense, this may also relate to the role of bloodflow in the formation of focal intimal hyperplasia in known vein grafts,including inducement of eddy blood flow at locations of graft-hostdiameter mismatch.

These considerations and deficiencies can be addressed with the variousstructures and methodologies of the present invention in which a veingraft is provided that exhibits compliance properties mimicking those ofhealthy arteries. Radial expansion and contraction of the graft ispermitted in a manner that mimics the radial expansion and contractionof an artery to at least closely approach the desired result in whichthe vein graft, its connections to adjacent arterial ends or stumps, andthe adjacent arterial portions tend to expand and contract in a similarmanner, to thereby substantially avoid anastomotic compliancemismatches. This is accomplished through the use of a flexible,resilient, generally tubular external support that engages the ablumenalsurface of a vein segment carried within the support, the support beingso fabricated as to functionally provide the graft with the complianceproperties of an artery.

Compliance Properties

As noted earlier, compliance is the ratio of the diameter change of avessel in the radial direction to a given change in vessel pressure, andthe values for compliance referred to below result from dynamic, invitro testing. Compliance values are reported here as percentage changesin the internal diameter of a vessel per a 100 mm Hg change in vesselpressure, as measured in the range of normal blood pressures, that is,from about 80 mm Hg to about 120 mm Hg. In the laboratory, it isconvenient to measure compliance through the use of an elongated balloonstructure over which a candidate tubular support is positioned.Distilled water at about 37° C. is pumped into the balloon to cause itto inflate, and the pressure within the balloon is cycled between 0 mmHg and 140 mm Hg at a frequency of about 72 cycles per minute to mimic anormal pulsatile blood flow. The change in internal volume is measuredbetween 0 mm Hg and 140 mm Hg to provide pressure/volume data. From thisdata is subtracted the pressure/volume data resulting from repeating theprocedure with the balloon alone, and from the resulting pressure/volumedata the percentage change in the internal diameter of the tubularsupport between 80 and 120 mm Hg can be calculated. It is convenient toexpress this radial compliance value as %/100 mm Hg.

The compliance of an implanted venous graft may be measured in vivothrough the use of ultrasound techniques in which the vein graft isvisualized in a cross-sectional view and the dimensional change of thevessel with varying blood pressure is recorded for at least one andusually a number of cardiac cycles. The cross-sectional lumenal area ofthe vein graft is measured for the smallest cross-sectionalconfiguration and the largest cross-sectional configuration for onecardiac cycle. The smallest cross-sectional configuration of the veingraft lumen is associated with diastolic blood pressure whereas thelargest cross-sectional configuration is associated with systolicpressure. The cross-sectional lumenal area values for diastolic andsystolic blood pressure are used to calculate the lumenal diametervalues and the vein graft compliance. Compliance values of a venousgraft measured in vivo often are slightly larger that the compliancevalues measured in the laboratory, and the compliance values referred toherein are laboratory values resulting from the in vitro measurementsdescribed above.

FIG. 2 is a sectional representation of vascular tissue useful forillustrating the relation of the natural arterial structure with theprosthetic venous graft structure of the invention. The naturaladventitial layer 95 of an artery 98 is comprised of two main tissuetypes that contribute to the mechanical properties of the naturalartery, namely elastin and collagen. The mechanical properties of thesetwo soft tissue components are described in Table I below: TABLE IMechanical Properties of Soft Tissue Components Soft Tissue ElasticModulus (Pa) Max Strain (%) Elastin 4 × 10⁵ 130 Collagen 1 × 10⁹ 2-4

As shown in the above table, these two soft tissue types have largedifferences in mechanical properties. Elastin is very elastic, andcollagen is very stiff in comparison. These two tissue types arecombined in the adventitial layer to produce a non-linear elasticresponse. As shown in FIG. 3, the combined effect of the characteristicsof elastin 101 and collagen 104 (having a greater role at higherstrains) results in a non-linear response curve (shown loading at 135and un-loading at 137) within the physiological pressure range of anatural artery between about 80-120 mm Hg. This characteristic ofpulsatile expansion and contraction of arteries requires fine mechanicalcompliance of any prosthetic graft, i.e., a close mimicking by theprosthetic device of the mechanics and timing of the natural arterydistending and reshaping under change in blood pressure.

From an engineering standpoint, the following relationships may behelpful from a design standpoint in producing venous stent grafts of theinvention. $\begin{matrix}{C_{d\quad} = {{\frac{\Delta\quad D}{{D_{diastolic} \cdot \Delta}\quad P} \cdot 100 \cdot 100}\quad m\quad{mHg}}} & (1)\end{matrix}$

in which C_(d) is compliance, P is blood pressure, ΔP is the differencebetween systolic and diastolic blood pressures, D is vessel diameter,and ΔD represents the diameter change between systolic and diastolicpressures.

The stiffness of blood vessels is stated as a stiffness index (β), andis a measure of the changes of curvature and diameter, stated as:$\begin{matrix}{\beta = {\frac{\ln\quad\frac{P_{systolic}}{P_{diastolic}}}{\frac{\Delta\quad D}{D_{diastolic}}} = {D_{diastolic}\frac{{\ln\quad P_{systolic}} - {\ln\quad P_{diastolic}}}{\Delta\quad D}}}} & (2)\end{matrix}$

A related characteristic of blood vessels is that of elastic modulus(K), which is considered a measure of stiffness, and is stated as:$\begin{matrix}{K = {\frac{{V_{diastolic} \cdot \Delta}\quad P}{\Delta\quad V} \propto \frac{{D_{diastolic} \cdot \Delta}\quad P}{\Delta\quad D} \propto \frac{1}{C}}} & (3)\end{matrix}$

in which C is compliance, V_(diastolic) is the vessel volume per unitlength at diastole, and Δ V is the difference in unit volumes betweensystole and diastole. In terms of diametric compliance, as an example,$\begin{matrix}{K = {{D_{diastolic}\frac{P_{systolic} - P_{diastolic}}{D_{dystolic} - D_{diastolic}}} = {D_{diastolic}\frac{\Delta\quad P}{\Delta\quad D}}}} & (4)\end{matrix}$

FIG. 4 shows that the Elastic Modulus (K), as defined in the aboveequations, is proportional to the secant S₁ of the pressure-diametercurve PD₁, plotted on a linear scale (left y-axis in FIG. 4), betweendiastolic and systolic pressure. The slope,(P_(syst)-P_(diast))/(D_(syst)-D_(diast)), of the secant S₁ is a goodapproximation to the slope of the pressure-diameter curve PD₁ in thatpressure range. From the above equations for the Elastic Modulus (K) itcan be appreciated that the Elastic Modulus (K) is not equal to theslope of the secant S₁ but is proportional to the slope by a factorD_(diastolic). Compliance (C_(d)) is approximately proportional to theElastic Modulus (K) hence it is approximately proportional to theinverse of the secant S₁ of the pressure-diameter curve PD₁ betweendiastolic and systolic blood pressure.

The stiffness index (β) is proportional to the secant S₂ of thepressure-diameter curve PD₂ between diastolic and systolic bloodpressure when the pressure-diameter curve is plotted on a logarithmicpressure scale (right y-axis in FIG. 4). The slope of the secant S₂ is(In P_(syst)-In P_(diast))/(D_(syst)-D_(diast)) and is a goodapproximation to the slope of the pressure-diameter curve PD₂ in thatpressure range. It can be again appreciated, from the above equationsfor the Stiffness Index (β) that the Stiffness Index (β) is not equal tothe slope of the secant S₂ but is proportional to the slope by a factorD_(diastolic).

Compliance data of natural human vessels is categorized by vessel typeand by age of the vessel (i.e., age of patient). For example, a commoncarotid artery has about a 6.6%/100 mm Hg compliance value. The valuesfor a superficial femoral artery and a femoral artery are 6-10%/100 mmHg. A value for a saphenous vein, however, is about 4.4%/100 mm Hg,while an aorta ranges generally from about 20-50%/100 mm Hg, dependingon the location. Also, the lengths of grafts according to location inthe body must be considered, and substantial lengthwise variance ingraft lengths is not uncommon. It is also known that the diameter ofvarious arteries change over time, and this may have a significantimpact on overall compliance values. Returning to FIG. 1, line 80represents the pressure-diameter data that certain embodiments of venousgrafts of the invention seek to emulate, wherein the complianceproperties of a native artery (line 32) is closely mimicked.

Support Materials and Manufacture

The radially resilient support may be manufactured from any biologicallyacceptable material that possesses the ability to be shaped into atubular structure having the required compliance. Polymeric fibers maybe employed, such as polyurethanes, polyethylene terephthalate,polypropylene, and polytetraflouroethylene, and good results may beobtained through the use of wires of such metals as stainless steel andcobalt-chromium alloys. Polymeric fibers may be elastomeric polymers,e.g. polyurethane elastomers or composite fibers that act in an elasticfashion. Polymeric fibers may be “shrinking” polymers, where theshrinkage may be controllable, e.g., pressure-sensitive polymers. Wiresmade of shape memory alloys such as Nitinol may be used to advantage.Shape memory elements or filaments may be made of one or more shapememory materials as exemplified in Table II below, it being understoodthat this is not to be considered an exhaustive list. Also, any metal ormetal alloy may be coated with a polymer for improved biocompatibility,recognizing that the polymer may or may not be biodegradable. TABLE IIMaterials ALLOYS POLYMERS Ag—Cd Two component system based onoligo(Σ-caprolactone)dimethacrylate and N-butyl acrylate Au—CdPolyurethanes Cu—Al—Ni Polynorborenes Cu—Sn Poly(ether ester)sconsisting of poly(ethylene oxide) and poly(ethylene terephthalate)(EOET copolymers) Cu—Zn Ethylene vinyl acetate copolymers Cu—Zn—SiPolystyrene polybutadiene copolymer Cu—Zn—Sn Cu—Zn—Al In—Ti Ni—Al Ni—TiFe—Pt Mn—Cu Fe—Mn—Si

With respect to shape memory alloys, other design considerations includetemperatures, different diameters and radial compliance, shapetransformation dimensional changes, and wire thicknesses. Generally,shape memory alloys and shape memory polymers may have transformationtemperatures which are below physiological temperatures, i.e., 37° C.,to ensure self-righting responses. Preferably, transformationtemperatures will also be above room temperature to ensure that theshape memory material reinforcing does not need to be refrigerated forstorage purposes. Thus, the ideal shape memory transformationtemperatures will likely be between 21° and 37° C. This transition mayeither be a two-way or a one-way directional transition, with acurrently preferred embodiment including a two-way directionaltransition. The transition temperature range can either be a short, i.e.0.5° C., or a long transition temperature range, i.e. 10° C., where theshape is proportionally regained over this temperature range. Forexample, for a desired temperature transition to be 100% complete at 25°C. but with it starting at 20° C., then this would yield a temperaturerange of 5° C. The changes in radial diameter due to the shape memorymaterial experiencing transformation dimensional changes is preferablyin a range of from 5% to 30%.

An embodiment of a tubular support utilizing a shape memory alloy isillustrated in FIGS. 5 and 6. FIG. 5 shows an arterial reinforcementtubular support 77 formed of one or more shape memory material elements165. These elements are braided, but may also be knitted or woven, intoa generally tubular structure designed for placement around a portion ofa vein to produce an arterial graft. In this example, a shape memoryalloy is employed because of its so-called “superelastic” propertiesrather than its ability to undergo temperature-induced phase changes,although some phase change from austenite to stress-induced martensitemay occur. In FIG. 5, the braided tube is positioned on a hollow plasticstraw as representing a vein segment, and has been compressed axially toproduce an increase in diameter. By extending the braided tube axially,as shown in FIG. 6, the tube becomes reduced in diameter to providesupport to the vein segment.

The shape memory braided material shown in FIGS. 5 and 6, if used alsofor its phase transformation properties, may be supplied in a firstconfiguration (which may be in the martensite phase) which can be easilymanipulated to receive a vein segment 86 within the structure, and asecond configuration (shown in FIG. 6, which may be in the highertemperature austenite phase) which has a “remembered” narrower diameterconfiguration to provide support to the vein segment. The contact ofinner surfaces 170 of the structure with ablumenal surfaces 175 of thevein segment 86 is shown also in FIG. 7. The resilience of shape memorymaterials can be controlled by altering compositions, temperingprocedures, wire diameters, etc., so that a tubular support fashionedfrom this material may mimic (when combined with the minimal mechanicalvalues of a vein segment) the compliance values of a host artery inorder to optimize the venous graft-artery interaction. This aspect ofcompliance mimicking has components of expansion, recoil, timing, andtissue remodeling. In this example, the vein-stent compliance values arechosen to closely mimic those of a healthy native artery. Whereas theshape memory wires are shown as braided in FIGS. 5, 6 and 7, they mayalso be knit, and in fact the knit configuration appears to offercertain advantages.

Radially resilient tubular supports may be knit from metal wire, such asstainless steel and cobalt-chromium alloys. Metal wires ranging indiameter from about 25 to 150 micrometers are appropriate for knitsupports with diameters in the range of 35 to 50 micrometers beingparticularly useful, although larger or smaller diameters may beemployed as desired. For braided tubular supports, metal wires rangingin diameter from about 37 to about 170 micrometers are appropriate,although larger or smaller diameters may be employed.

Knitting procedures may be performed by known methods using, forexample, a LX96 knitting machine manufactured by the Lamb KnittingMachine Corporation. Favorable radial compliance and tubular dimensionalproperties may result from knitting the tubular structure in a mannerproviding loops that alternate in the circumferential direction betweenlarger and smaller loops, as shown in FIG. 21. In this Figure, smallerloops 250 are shown alternating circumferentially with larger loops 251.Such alternating loop sizes typically present themselves visually aslongitudinal stripes extending axially along the tubular support, as theadjacent loops of each size align in the longitudinal axis. Each closedend of the loop may be either rounded or generally square-shaped orvariations in between, and, the sides of the loop may turn outward, beparallel, or turn inward. The latter design has shown some advantage inacting like a spring and assisting in the stability of the overalldimensions of the tubular structure, and maintaining its compliancecharacteristics.

Other geometries for the loops in the knitted structure arecontemplated. Additional embodiments are illustrated in FIGS. 33-36. Thenumber of loops per circumference is determined by the number of needlesused during the knitting process. The number of loops per longitudinalunit length and the similarity or dissimilarity of circumferentiallyneighboring loops is controlled by various parameters of the knittingprocess. From experimental data obtained with various Nitinol knittubular structures, it is suggested that the mechanical and structuralproperties of the knit tubular structure are controlled to a certaindegree by the geometrical features of the knit mentioned above. Thesegeometrical features include, but are not limited to thesimilarity/dissimilarity of circumferentially neighboring loops andnumber of loops per longitudinal unit length.

Tubular knit structures with different degree of dissimilarity ofcircumferentially neighboring loops are shown in FIGS. 33-35. FIG. 33shows high degree of dissimilarity (highly uneven neighboring loops),FIG. 34 shows an intermediate degree of dissimilarity (intermediateuneven neighboring loops), and FIG. 35 shows a low/zero degree ofdissimilarity (even neighboring loops). FIG. 36 shows a knit structurewith larger loop length in longitudinal direction of the knit comparedto the structures shown in FIGS. 33 to 35. A larger longitudinal looplength results in a lower number of loops per longitudinal unit length,e. g. per inch in longitudinal direction.

FIGS. 37 and 38 illustrate the dimensional parameters used to specifythe geometry of a neighboring pair of loops, both with respect to thedissimilarity of neighboring loops and the longitudinal loops length.The dimensional parameters are measured digitally in macroscopicphotographs of the knit structures, such as shown in FIGS. 33 to 36.Since the tubular structure has a circular cross section, linearmeasurements in the photographs in transverse/circumferential directionof the knit deviate from the actual circumferential dimension. Thedeviation depends on, and varies with, the transverse distance from themid axis of the knit. Dimensional parameters subject to this deviationare all except those in longitudinal direction, i.e. L₁, L₂ and L_(s).The deviation of the transverse/circumferential parameters H₁, N₁, H₂,N₂ are numerically corrected after completion of the measurements. Thedeviation correction includes the outer diameter of the knit, transversedistance between the knit mid axis and the start and end point,respectively of each measurement. The table in FIG. 39 gives measuredand corrected dimensional parameters for Knits #1 to #4 (shown in FIGS.33 to 36) as an example.

The dimensional parameters measured on macroscopic photographs ofmanufactured prototypes can be employed to specify particular loopgeometries and to perform dimensional ‘quality’ control of prototypes ofthe same loop design manufactured in different batches. This may requirethe dimensional parameters to be measured in macroscopic photographs ofprototypes of the newly manufactured batch. These measurements wouldthen be compared with the measurements of the original prototype.Dimensional loop geometry measurements on existing tubular knitprototypes may also serve as basis for knits with modified loopgeometries, e.g. with a different degree of dissimilarity of neighboringloops or different longitudinal loop length.

Regardless of how the tubular support is manufactured, the knitted orbraided tubular support may then be subjected to crimping to providecrimps extending, for example, about the circumference of the tubularsupport (that is, in the manner shown in FIG. 17). One way of doing thisis through the use of an axially fluted mandrel that is inserted intothe tube and is pressed outwardly against a wall of the tube to forcethe wall against a complementary shaped outer female mold to bend theknitted or braided wires and to form a circumferential crimp, the crimpresulting from each flute or raised ridge of the mandrel extendingaxially of the support.

A compliant venous graft using various metals or polymers for thetubular support may be provided in several ways. Embodiments may beadvantageously provided in knitted form. FIGS. 8 and 9 show material 165in a braided configuration, and FIGS. 10 and 11 show material 165 in aknitted configuration. Mechanical characteristics of the tubular supportmay be enabled by the type of shaping and relational structures formedon the elements making up the knit or braided structures. It iscontemplated that a technique involving crimping of the material 165 toachieve angular crimps (shown in FIGS. 12 and 13), formed prior to thebraid or knit construction, and rounded crimps (shown in FIG. 14) mayprovide acceptable results. Crimping techniques that may be appropriatewith pre-knit configurations are shown in FIG. 15 (angular crimps) andFIG. 16 (rounded crimps). Another technique for achieving certainperformance characteristics of braided or knitted shape memory materials165 is to perform crimping after braiding or knitting, i.e.post-braiding or post-knitting. FIG. 17 shows one embodiment of material165 formed in a braided configuration and having a post-braided crimpingoperation applied to form a crowned pattern to achieve desired crimpcharacteristics.

Crimp angle and pitch density may be important variables in certainembodiments of the current design of the tubular supports. It isunderstood, however, that certain advantages of this invention may stillbe achieved without use of crimping. Ranges of crimp angle arepreferably from about 10° to 85° to the line of lay of the reinforcingwire or braid. The crimp size may vary from 0.01 to 5 mm in length. Itis desired that the braid or helical wires have a pitch angle that mayvary from about 5-85° to the axial length of the vein graft.

Applicants have identified certain crimping techniques that relate tocrimping either before or after braiding or knitting. For example, inpost-braid crimping the material braids are produced according toexisting techniques, after which macroscopic longitudinal crimping isimparted to the tubular mesh using a post-braid crimping tool. However,according to the material and specific configuration of the stent, ifthe post-braid crimping of braided tubes does not achieve sufficientcompliance then alternate methods are possible. One example is to effectpre-braid crimping, thereby setting the memory of a shape memorymaterial in a crimped configuration and subsequently straightening thematerial before braiding. The crimp is thus induced upon exposure tophysiological temperatures.

In other embodiments, rather than having a knit or braided structure,the graft may include a plurality of rings 400 that are connectedtogether to form a tubular structure, as shown in FIGS. 40-43. The rings400 may be made from any suitable material, such as the metals, alloys,and polymers discussed above. As shown in FIGS. 40 and 41, the rings 400may be interlinked together so as to form a continuous chain 402. FIG.40 shows only a portion of the structure in an unrolled state, andillustrates a configuration in which a central ring 404 connects threerings 406 on one side thereof with three rings 408 on the other sidethereof. Such a connection may be used throughout the tubular structure.FIG. 41 also shown only a portion of the structure in an unrolled state,and illustrates a configuration in which a central ring 410 connects tworings 412 on one side thereof with two rings 414 on the other sidethereof.

In another embodiment, rather than being interconnected in a chain-linkfashion, the rings 400 may be connected at their outer surfaces 416, asshown in FIGS. 42 and 43. The rings 400 may be welded together, wherethe rings are made of a metal or alloy material, or may be connectedwith any suitable adhesive, especially in embodiments that include ringsmade from a polymer material. In the embodiment illustrated in FIG. 42,the rings 400 are connected so that no single ring contacts more thanfour other rings, i.e., the rings are connected at surfaces that aredisposed 90° from each other. In the embodiment illustrated in FIG. 43,the rings 400 are connected so that no single ring contacts more thansix other rings. This configuration provides a more tightly packedstructure, as illustrated in FIG. 43.

Of course, any suitable number of rings may interconnected or connected,so long as the resulting structure provides the compliance andperformance properties of the grafts and stents discussed herein.Properties of the resulting stent may be altered by varying the patternof the rings, the internal diameter of each ring, the thickness of eachring, etc. Compliance of the resulting stent may be achieved by thedeformation of the rings into oval shapes upon an application of force.It is also contemplated that the rings within a single stent may havedifferent properties, e.g, different diameters, thicknesses, shapes, andmaterials, and that the rings may be combined with the knitted patternsdiscussed above. The illustrated embodiments are not intended to belimiting in any way.

In certain embodiments, it is appropriate to provide for “jump” grafts,or “skip” grafts to communicate a stented graft with another vessel. Toaccommodate such grafts, an opening is made in the resilient, externaltubular support of a compliant graft of the invention so that a portionof the vessel wall itself is exposed through the opening to enable ajump graft to be attached at that location. It is desirable to providefor such openings in the support wall prior to assembly of the compliantgraft. When the tubular support is made of a shape memory alloy, such asNitinol, an opening in the mesh may be made by supporting the mesh on anappropriately shaped mandrel and gently moving the fibers forming themesh away from what is to be the center of the opening. A pin or othersupport is placed in the opening to keep it open, the pin being held andsupported by the mandrel. The tubular support, constrained in thisshape, is subjected to a heat treatment, e.g., in the 500° C. range, fora short period of time and then cooled. The resulting tubular support,in its austenite phase, exhibits the usual super elasticity associatedwith Nitinol and other shape memory alloys, and the opening thusprovided in the wall of the tubular support remains open and accessiblefor formation of a jump graft. By selection of an appropriately shapedand sized tubular support with a pre-formed access opening, a surgeonmay produce a graft prosthesis having an opening in the wall of thetubular support positioned where desired for formation of a jump graft.

The external tubular support adjusts the mechanical and geometricalproperties of the vein graft to match or mimic healthy arterialproperties and therefore adapt to the arterial pressure of the hostartery. Accordingly, this results in substantial matching of the lumenof the vein graft and the host artery, the substantial matching ofcompliance of the vein graft and the host artery, and substantialmatching of the radius to wall thickness ratio (r/wt) of the vein graftto the host artery. As noted above, optimization of the vein-stentcompliance should ensure that the vein-stent graft mimics the behaviorof arteries regarding the non-linear stiffening with increasing diameterdue to elevated blood pressure, “locking” at a maximum pressure, andthen demonstrating dynamic recoil in a timely manner.

When venous grafts utilizing knit or braided tubular supports are cut atangles suitable for end-to-end anastomoses, either at generally rightangles or in scallop-like shape, the ends of the supports may experiencefraying (see, for example, FIG. 17). Certain methods and structure arehelpful to eliminate such fraying. In one embodiment, adjustable rings210 of bioabsorbable or biodegradable material are placed generallycircumferentially around a portion of the material 165, and in contactwith external surfaces 217, as shown in FIG. 18. The number of rings maybe varied as needed. The location of the rings may be adjusted to theposition of anastomoses where vein and tubular support need to be cut.The cut or suture may be carried out through the ring, and the ring maybe absorbed or degraded over a predetermined time, as desired.

Another embodiment of a structure to prevent fraying of a knit orbraided tubular support when it is cut is the use of polymer coating forthe fiber mesh. This feature may also provide the benefit of preventinggluing of joints and contact zones of elements of the stent. However,use of the radially compliant tubular support as a reinforcing structuremay advantageously involve bonding of the ablumenal surface of a veinsegment to confronting internal surfaces of the support. This attachmentor connection may be accomplished through the use of a glue or othermaterial having adhesive or connecting characteristics. In oneembodiment, a fibrin glue or other material having adhesive orconnective characteristics may be sprayed on designated portions of thevein (as exemplified at 283 in FIG. 20) and/or the tubular support. Thefibrin glue may be an autologous fibrin glue or autologous platelet gel,as described in U.S. Pat. Nos. 6,444,228 and 6,596,180, which are bothincorporated herein by reference in their entireties.

The adhesives, whether synthetic or natural, may be applied by spraying,brushing, sponging or dripping the material onto the stent/graftconstruct, or applied by the applicator. To prevent fraying of a knittedor braided structure, the stent may be pre-coated, by dipping, spraying,brushing, etc., with an elastomeric material that binds the individualwires together in such a manner that it prevents fraying at ends/cutedges while maintaining compliance. The material can be of synthetic(polyurethane, silicone, polyvinyl alcohol) or natural origin (fibringels, collagen gels, albumin etc). As an extension of this embodiment,the material used to prevent fraying may further effect adhesion of thestent to the vein/graft by incorporation of reactive groups capable ofreacting with the graft/vein ablumenal surface. It is also contemplatedthat selective spot welding of wire and material patches may be used toprevent fraying of a knitted or braided structure.

In embodiments pertaining to adhesion and antifraying, thegels/glues/adhesives that are used may further contain bio-active agentsthat are released (either by diffusion from a non-degradable adhesive orby release from a degradable one) to effect a desirable biologicalresponse. For example, growth factors may be incorporated to stimulateand increase the vascularization (formation of additional vasa vosora)that in turn result in improved outcomes. In an embodiment, steroids maybe released to minimize the foreign body reaction to the stent/adhesivematerial(s).

Another embodiment includes placement of a material on designatedportions of the lumenal surfaces of the tubular support so as to providethe characteristics of contact adhesion and/or bonding when theseportions contact the vein. However, the glue or other material must notinhibit the function of the tubular support. It is desirable that thecontact of the tubular support with the ablumenal vein segment surfacebe reasonably uniform along the length of the support, and that regionsof much higher force of the support against the ablumenal wall of thevein be avoided.

Performing the anastomoses of small-diameter unsupported vein grafts inthe coronary position is complicated by the tendency of the free end ofthe vein to collapse on itself, thereby obscuring the lumen and makingit difficult for the surgeon to identify a suitable position for theplacement of sutures. The application of an external tubular support(referred to sometimes herein as a stent) on a vein graft potentiallyfurther complicates the suturing, as the collapsed vein is situatedinside (and at least partially obscured by) the non-collapsed stentmaterial. By attachment of the ablumenal surface of the vein to thelumenal surface of the stent, however, the stent offers support to thevein to prevent it from collapsing, and this is particularly the casewhen stents formed from shape memory alloys are employed. As notedabove, adjustable rings 210 and various adhesives may be employed tobond the confronting surfaces of the vessel and the supporting tubularstructure together.

Attachment of the vessel surface to the tubular support stent can beachieved in various ways. In one embodiment, a covering gel is employedthat attaches to the vessel wall and surrounds and entraps the stentwires, thereby attaching the stent to the vessel, this embodiment beingschematically depicted in FIG. 24, the tubular support fibers beingshown at 320, the vessel wall at 322 and the gel at 324. Examples ofsuch gels are synthetic gels (such as modified polyethylene glycol,polyvinyl alcohol, acrylic gels, etc) and biological gels (such asfibrin, gelatin, and albumin glues). In another embodiment shownschematically in FIG. 25, an adhesive glue 326 such as a cyanoacrylateattaches the stent to the vessel ablumenal wall, the glue adhering toboth the vessel and the stent.

In another embodiment, shown schematically in FIG. 26, individual fibers320 of the stent are coated with an adhesive material 328 containinggroups reactive to the vessel tissue. This material may be eitherdirectly applied on the metal wires (FIG. 26) or on a polymeric coatingwith which the wires are pre-coated. This material coating, appliedoptionally over a polymer coating on the wires, may also be employed inother embodiments, for example, those shown in FIGS. 24, 25, 27 and 28.

In a preferred embodiment, a sleeve is placed over the assembled stentedvessel. As schematically depicted in FIG. 27, the sleeve 330 mayprimarily offer mechanical support for the stent to prevent fraying ofcut edges, in the manner discussed above in connection with FIG. 18,while having minimal if any effect on the mechanical properties of theassembly, such as compliance. Referring to FIG. 28, a sleeve maycomprise an adhesive tape 332 having an elastic backing material 334bearing an adhesive material 336 having a consistency enabling it topenetrate between fibers of the tubular mesh and adhesively contact theablumenal surface of the vessel. The adhesive may have the consistencyof a gel. The elastic material may be a fabric, and may be formed from avariety of materials that are inherently elastic (e.g. polyurethaneelastomers) or materials that are not elastic by themselves, but may actin an elastic fashion due to the fact that they are coated with orentrapped in an elastic gel-like material that constitutes the adhesiveportion of the tape. Crimped fibers made from polyesters (PET) ordegradable materials such polylactic acid (PLA) or polyglycolic acid(PGA) or copolymers thereof may also be used for the elastic material.

The adhesive material 336 should be of sufficient cohesive strength andadhesive strength to the vessel wall by virtue of mechanicalinterlocking and/or covalent chemical binding to attach the stent to thevessel during normal handling during implantation. As the vessel tissuecontains both nucleophilic (amino, thiol and hydroxyl groups) andelectrophilic (carboxyl) groups, the adhesive may employ a number ofchemical groups capable of reacting with the vessel wall. Aldehydes,acyl chlorides, activated esters, isocyanates or carboxylic acids (plusactivators such as carbodiimides) are examples of compounds capable ofreacting with nucleophilic groups on the tissue, and alcohols and aminesmay be employed to react with the electrophilic carboxylic acid groupson the tissue (in the presence of an activator, e.g. a carbodiimide).

In general, the adhesive may be composed of synthetic polymers, in theirswollen or unswollen states. Gel-like characteristics may be imparted bythe adhesive material itself, or by swelling the material with a solventor a plasticizing agent, such as water. Gels offer the advantages ofhaving viscoelastic properties that may simulate the mechanics ofvessels to some extent, and of being capable of deforming to facilitatecontact and binding of the gel to the tissue through the gaps in thestents. In addition, gels may contribute to the strength of the bondbetween the vessel wall and the stent by mechanical interlocking.Adhesives may be non-degradable cross-linked materials such aspolyethylene glycols, polyimines, polyacrylates (e.g., polyacrylic acid,polyacrylamides, poly(hydroxyethyl methacrylate), and co-polyacrylates.Degradable and/or resorbable adhesives may include multifunctionalpolyethylene glycols containing degradable end groups and/or crosslinkedwith degradable crosslinkers, and non-crosslinked or lightly crosslinkedpolyvinyl alcohol.

As noted above, the adhesive material desirably is functionalized withgroups capable of reacting with the vessel tissue. For example, anadhesive may comprise crosslinked polyacrylic acid gels functionalizedwith aldehyde groups (e.g., via a diamine bridge) capable of reactionwith tissue amines. As another example, a polyvinyl alcohol, the degreeof hydrolysis, molecular mass and tacticity (and thus crystallinity)have been adjusted to render the adhesive slowely dissolvable in vivocan be appropriately functionalized with groups reactive toward tissuegroups.

Although the adhesive tape 332 may be applied as desired to the stentedvessel, a preferred method involving particular stented vessels having asupportive tubular structure derived from a shape memory materialutilizes a pre-assembled clip comprising a disposable elastic metal clipand a segment of the adhesive tape, as schematically depicted in FIGS.29 A through D. Referring to FIG. 29A, the metal clip 340 is shaped toprovide an opening for reception of the stented vessel, and may, forexample, be generally “C” shaped. The clip contains within it a sectionof the adhesive tape 332, the adhesive surface 336 facing the interiorof the clip and the elastic backing adjacent the inside surface of theclip. As desired, the assembled clip may be positioned within the jawsof a pliers-like applicator 342 (FIG. 29B). In use, a section 344 of astented vessel is inserted into the opening of the clip as shown in FIG.29B. The jaws of the applicator 342 are closed on each other, thestented vessel becoming temporarily flattened as shown in FIG. 29C andthe adhesive penetrating between fibers of the stent to adhesivelycontact the ablumenal surface of the vessel. The jaws of the applicatorare opened, enabling the section of the stented vessel to resume itssubstantially circular cross-sectional configuration. The metal clip isremoved, and excess tape is removed from the edges to provide thestented tube structure schematically shown in FIG. 29D.

An alternative method involves the employment of a tape dispenser thatcontains a continuous roll of the tape, sections of the tape beingsevered from the roll and applied to the stented vessel by hand.

The application of the tape to the stented vessel can be performed at aposition required by the surgeon according to the implant position. FIG.30 shows how a taped section 350 of a stented vessel may be cut toprovide a biased opening that may be sutured to, for example, a coronaryartery. The taped section is cut so that the resulting open end isconfigured to conform generally to the ablumenal surface of an artery orother vessel to which the stented vessel is to be attached. FIG. 30shows a cut being made generally along the dashed lines to providesegments 352, 354. FIG. 31 shows how the bias-cut taped end of thesegment 352 may be sutured to a length of artery 356, and it will beunderstood that the artery has a surgically prepared, axially extendingslit (not shown) providing an opening through its wall about which thesegment 352 is sutured to communicate the graft with the artery.

By appropriately cutting the taped portion, the end of the segment canbe shaped to conform as needed to the external contour of variousvessels to which it is to be attached. For example, a graft may beshaped to make an appropriately angled (e.g., 45 degrees) juncture witha coronary artery, the artery and the stented graft segment lyinggenerally in the same plane. In another example, the stented graft maycross over a vessel to which it is to be attached. In this“cross-overjump” configuration, slit-like openings are provided in thetaped section of the graft segment and the vessel, and the taped sectionand the vessel are sutured together about their confronting openings andto establish fluid communication through the openings. The slit-likeopenings may be made such that they are approximately parallel as theyconfront each other. In yet another example, the stented graft and avessel may be positioned substantially parallel and in contact with eachother. In this “longitudinal jump” configuration, slit-like, desirablyaxially extending openings are made in the taped section of the stentedgraft and in the vessel, and suturing is performed as above.

Applicants have further recognized the need for a device to facilitateassembly of the radially compliant tubular support and a vein segment.It is desirable that such an applicator should not obscure the stentlumen, and that it should allow for easy insertion of the vein. It isfurther recognized that a design whereby diameter is increased by lengthcompression, as in a braided configuration, would allow easy slipping ofthe tubular support over a vein. FIG. 19 illustrates this concept incombination with an applicator 279 to apply the braided support 284 to avein 86. This longitudinal braid contraction phenomena (shown earlier inFIGS. 5 and 6), and which must be carefully managed at the time ofjoining the vein to the stent, is likely quite useful to achieving thegoals of an applicator 279, as noted above. This applicator may alsofacilitate placement of anti-fraying rings 210. In one embodiment, themethod of using the applicator comprises the steps of: providing themeans of longitudinally contracting a stent; holding the stent in thecontracted position with increased stent diameter resulting; inserting avein into the stent lumen; and distending the stent longitudinally whilethe vein is inserted simultaneously until the stent is slipped over thedesired portion of the vein. Further design considerations must ensurethat the stent will not be fixed to the vein in a longitudinallyover-distended or contracted state, so as to ensure that thepredetermined mechanical stent properties remain viable.

FIG. 20 shows an embodiment in which a tubular support 185 is receivedalong the outer surface of an applicator 281 having an internal passage,and, while passing the vein segment 86 from within the applicatorpassage, the tubular support is drawn onto the ablumenal surface of thevein segment. The applicator here may be a thin walled tube resembling asoda straw.

FIGS. 22A and B show the end of a syringe 300 having a mechanism 302 forengaging and immobilizing the end of a blood vessel and the surroundingtubular support. The mechanism may be any mechanism adapted to connectto the ends of open tubes, and one such mechanism may include an outertubular portion 301 within which is received an end portion 304 of ablood vessel, the mechanism including an axially movable internaltapered mandrel 305 that is inserted in the vessel. Axial movement ofthe mandrel (to the left in FIG. 22) pinches and captures the end of theblood vessel between the mandrel and an introducer sheath carrying atubular support, the sheath and tubular support together being shown inFIG. 22A as 303.

Referring to FIGS. 22B and 23, in one assembly method, the distal end304 of the vessel is supported by the mechanism 302 with the proximalend of the vessel being attached to a cord 306. The tubular support 308(FIG. 23) is carried exteriorly of and is supported by an introducertube 310. A plug 312 sized to engage the interior wall of the introducertube is attached to the proximal end of the vessel. The introducer tube310, bearing on its outer surface the tubular support 308, is drawn overthe cord 306, the plug and the vessel segment, the cord being maintainedunder tension to facilitate sliding the introducer tube over the vessel.The end of the tubular support is gripped by the mechanism 302 (FIG.22A). Thereafter, the introducer tube is withdrawn proximally away fromthe mechanism 302. By frictionally engaging the interior surface of theintroducer tube, the plug 312 exerts a gentle longitudinal tension onthe proximal end of the vessel, causing the vessel to shrink somewhat indiameter. As the introducer tube is withdrawn, the tubular support 308comes into contact with the ablumenal surface of the vessel. Once theintroducer tube has been completely withdrawn, the gentle longitudinaltension on the vessel is relaxed and the vessel itself seeks to returnto its original larger diameter to thereby more closely contact thetubular support.

The plug may be made of any appropriate material that frictionallyengages the interior wall of the introducer tube as it is withdrawn, anda soft urethane rubber plug, for example, may be employed.

Although assembly of the stented graft has been described as occurringaway from the vein or artery to which it will eventually becomeattached, in practice, such assembly can be affected by attaching oneend of the harvested vessel segment to the artery or vein with a cordattached to its other end. The mesh tubular support is then urged gentlyover the ablumenal surface of the vessel while maintaining gentletension on the cord until the support is positioned where desired.

It is desired that the support be applied to a vein at a predeterminedlength which is associated with a particular desired compliance. Alength-defining support feature or system should ensure a predeterminedsupport length. This is particularly true with respect to braidedsupports, and perhaps less important with knit supports in which radialresilience is less dependent upon the amount to which the support isextended axially.

In a braided support, and to a much lesser extend in a knit support,compliance and related mechanical properties are linked to the supportlength through the pitch angle. Imparting a change in length results ina change in pitch angle and compliance. However, the compliance of thesupport is a mandatory characteristic which is optimized, as notedabove, to mimic the compliance of a normal healthy host artery. Whenapplying a support to a vein segment, it is important to accuratelyaccommodate the predetermined tubular support length, even afterlongitudinal contraction of the support for the attachment of thesupport to the vein.

With braided, and to a much lesser extent knit supports, axial supportlength may be controlled, for example, through the use of an axiallyextending, relatively inextensible element, (as for example the thread78 in FIG. 7), that restrains the tubular support from unwanted axialextension. The thread may be woven through the support mesh and may befastened, as by welding, to the various wires that the thread encountersalong the length of the support so that as the support is stretchedaxially, the extent of axial elongation is controlled by the thread asthe thread is drawn taut. Moreover, this feature may enable a length ofthe tubular support to be divided into portions of appropriate length,with the permitted axial extension of each portion controlled by thesection of thread within it. As presently envisioned, a vein segment maybe sheathed in a tubular support as discussed in detail above, with theintent of cutting out a smaller segment of the resulting venous graftfor use in the surgical replacement of an artery, and the venous graftthat is thus used will have vein and tubular support ends that arecoextensive.

Various generally tubular external wire mesh supports were fabricatedfrom metal wires by braiding and by knitting, some being crimped andothers not, and the diametrical compliance of each was measured usingthe in vitro diametrical compliance testing outlined above. The measuredcompliance values were dependent upon many variables, including wiresize, tightness of braid or knit, etc. The following values listed inTable III were obtained: TABLE III Compliance Values Compliance Design%/100 mmHg A Braided Non-crimped 0.9 B Braided Crimped 5.6 C BraidedCrimped 1.8 D Knitted Non-crimped 3.4 E Knitted Crimped 7.9 F KnittedCrimped 8.0 G Knitted Non-crimped 10-21 H Knitted Non-crimped  9-21 IKnitted Non-crimped   16->30 J Knitted Non-crimped >30 K KnittedNon-crimped 10-16 L Knitted Non-crimped 21-29 M Knitted Non-crimped22-28 N Knitted Non-crimped >30 O Knitted Non-crimped 10-15 P KnittedNon-crimped  9-11 Q Knitted Non-crimped 13-24 R Knitted Non-crimped >30

A surgical procedure is proposed for use of the graft disclosed herein.This procedure, which may also be viewed as a simple method forplacement of a venous reinforcement structure, includes, in thisexample, application of the compliant external tubular support duringthe procedure of vein excision. In many instances, vein excision isconsidered a standard part of a surgical operation, which is usuallydone by an assistant at a time when the surgeon is still in thepreparatory phase of the operation. When an autologous blood vessel suchas a segment of the saphenous vein is harvested for use in accordancewith the invention, it may contain side branches that need to be removedbefore the vein is enclosed within the tubular support. Closure of theopening that remains after removal of a side branch can be accomplishedin several ways. Surgically placed sutures or small clips such as Ligaclips may be used to ligate small sections of branches. However, when abranch has been removed flush with the ablumenal Surface of the vessel,it is desirable to close the resulting opening in the vessel by suturingto reduce interference with the tubular support. Purse string suturesare appropriate. To avoid undue narrowing of the lumen of the vesselwhen a purse string suture is pulled tight, the ends of the purse stringsuture are preferably tightened by pulling them in the longitudinaldirection, i.e., axially of the vessel, rather than in a directiontransverse to the longitudinal direction of the vein.

In one embodiment, an initial step includes dissection and freeing of asaphenous vein. The saphenous vein is dissected free in a standard way,leaving it in situ while ligating and cutting off its branches asdiscussed above. The second step involves testing for potential wallleaks of the vein. In order to test the isolated saphenous vein forpotential leaks, it is normally cannulated distally and cold heparinisedblood is injected while the proximal end is occluded. This inflation ofthe vein (using old techniques) with a syringe creates pressures of upto 350 mm of Hg and is often a main reason for traumatic damage of thevein wall. Therefore, a pressure limiting mechanism may be positionedbetween the vein cannula and the syringe. The external tubular supportcannot be applied yet because leaks in the vein wall need to be freelyaccessible for repair. Therefore, no over-distention protection isplaced around the vein yet, necessitating the limitation of theinflation pressure to a level suitable for detecting any leaks ofconcern but less than a level deemed to cause unacceptable dam age, suchas, for example, in one embodiment, 15 mm of Hg, the pre-maximaldilatation pressure for veins. The tissue remodeling functions ofapplicants' invention become more critical in view of the importance ofleak testing and the reality of possible damage to the intimal layer inthe vein during even the most carefully performed leak testing.

The next step involves assembling the harvested vein segment and anexternal tubular support of this invention. In this step, the tubularsupport (typified here as a knit support) is mounted on a tube orstraw-like applicator within which is positioned the vein segment. SeeFIG. 20. The tube or straw-like applicator may be pre-treated with alumenal slip coating to minimize friction between the vein and theapplicator as the applicator is positioned over the vein segment,thereby minimizing stretch injury to the vein. The applicator is thenremoved axially, leaving the support and vein in contact to form thevenous graft. Over-extension of the tubular support is prevented using alength-limiting central thread or other means, as described above. Asrequired, the vein segment is then inflated under arterial bloodpressures of 120 mm of Hg, causing it to contact the tubular supportinner lumenal surfaces. In certain embodiments, an adhesive securing thetubular support to the vein will ensure that the vein does not collapseduring the surgical procedure when no internal pressure is applied.Again, it should be recognized (without limitation) that this is one ofseveral ways to accomplish the above objectives.

The following sequence may occur at this or another time during theprocedure. One of the external anti-fraying rings or cuffs is slid tothe end of the vein, and a typical double-S-cutting line is used toprepare the end for the first anastomoses. The thin cuff preventsfraying of the tubular support and also gives the vein tissue andtubular support a uniformity which makes the surgical process ofstitching the vein to the host artery in an end-to-side fashion mucheasier. Another thin anti-fraying ring is then slid down from theapplicator to a position where either a side-to-side anastomoses for asequential graft is being performed, or where the vein is being cut atan appropriate graft length. The half of the sliding cuff which remainson the original vein will make the process of the anastomoses to theproximal host artery much easier. In the case of a coronary arterybypass graft, for example, the end of the remaining vein protected bythe other half of the cuff is used for the next distal graftanastomoses.

Although the invention has been described primarily in connection withthe use of autologous blood vessels to form a stented graft as areplacement for diseased vessels, the invention has other uses as well.External stent structures can be used to allow repeated access to thevascular system for administration of medicines or nourishment or theadministration of dialysis procedures. Exterior access grafts may besubject to frequent penetration by hypodermic needles, and it isdesirable not only that the vessel walls of the graft retain theircapacity to be punctured many times, but also that the wounds thusformed in the vessel wall self-seal to a large extent. In addition, forexample, the stent structure on its own may be added to veins toincrease competency of venous valves.

In accordance with the invention, an external stent structure 360 asshown in FIG. 32 may be produced from a blood vessel segment harvestedfrom human or non-human animals, such as segments of bovine or porcinearteries, the vessels having been treated, as by known aldehyde fixingor decellularization. A flexible, resilient tubular support of the typedescribed herein may be placed over the resulting venous or arterialvessel segment to provide a bioprosthetic access graft to enable readyaccess to the cardiovascular system. For this purpose, one may form abioprosthetic device utilizing a suitably treated bovine or porcineartery or vein segment by placing the segment within an elastic,resilient tubular support sized to contact and squeeze against theablumenal surface of the vessel when the vessel is filled with blood, aswhen it is being used as a shunt between an artery and a vein of apatient. In this manner, once a hypodermic needle has been inserted andwithdrawn from the wall of the vessel, the elastic, resilient tubularsupport tends to exert a squeezing force on the vessel, forcing theedges of the wound to close upon one another.

FIG. 32 is a photograph showing an access graft formed from adecellularized porcine artery segment 362 contained within a knitted,resilient tubular support 364. This figure illustrates a puncture wound366 formed by a syringe 368. The edges of the wound are brought togetherby the squeezing action imparted by the knitted support. It may be notedthat the knitted support, or braided support if desired, may be madewith loops or openings 370 that are sized to receive a hypodermic needleof the desired diameter. For hypodermic needles commonly used fordialysis procedures, for example, needle sizes, and hence the size ofopenings between fibers of the tubular support, may be in the range ofabout 1.8 and 2.1 mm.

As structures have become increasingly complex, not only in design butalso in the range of material use, pure analytical methods have begun tofail in describing the behavior of such structures. Due to thescientific challenge of closely matching a vascular graft of the typedescribed herein to a host, analytical methods are rendered somewhatobsolete. Development of a prosthetic vascular graft which mimics themechanical requirements and dynamic compliance of a normal healthyartery is made possible, however, with certain old tools, particularlycut and try methods in which incremental changes are made to thematerial or structure of the tubular support to modify its complianceproperties, and the resulting properties are used to guide furtherchanges. Empirical data or constitutive equations and mathematicalanalyses may be used for certain features. Alternatively, the use ofnumerical modeling with such tools as, for example, Finite ElementModels and Methods, relying on continuum mechanics, along with certainother tools makes this level of customization feasible.

Results of pre-clinical studies with various designs and sizes of astent for a baboon vein, as discussed in further detail below, haveshown that the inner diameter of the stent relative to the outerdiameter of the vein is a factor for the adequate performance of thestent and graft. Hereinafter, diameter of the stent refers to the innerdiameter of the stent, and the diameter of the vein refers to the outerdiameter of the vein.

The results have shown that under-sizing of the stent (i.e. the stentinner diameter is smaller than the vein outer diameter) provides maximumbenefit of the stent for the graft performance, whereas over-sizing ofthe stent (i.e. the stent inner diameter is larger than the vein outerdiameter) provides a graft performance and biological response that isrelatively poor, and possibly even worse, than in non-stented veingrafts.

Taking this into account, as well as the fact that the outer diameter ofa saphenous vein varies along the length of the vein, it has been foundthat the inner diameter of the stent: 1) should match the smallest outerdiameter of a particular vein at any location along the harvestedlength, and 2) may be smaller, but should not be larger, than thesmallest vein outer diameter. In addition, segments of a saphenous veinwith an outer diameter larger than the minimum diameter of that vein maybe “downsized” by the stent chosen according to method discussed herein.The terms “downsized” and “downsizing” as used herein refer tosituations in which the stent is used to change the outer diameter ofthe vein to a smaller diameter. It has also been found that downsizingis desirable up to a certain, maximum allowed, degree, i.e., percentageof the maximum vein outer diameter.

The stent inner diameter to match a particular saphenous vein should beselected according to the minimum and maximum of the vein along theharvested length. Additional criteria may also be used to select a stentof an appropriate size for the selected vein.

The maximum degree of downsizing of veins by stents according toembodiments of the present invention during certain pre-clinical studieswas approximately 40%. The degree, or percentage, of downsizing may beexpressed as being equal to (outer diameter vein—inner diameterstent)/(outer diameter vein)* 100.

Based on the analysis of the outer diameter of 50 human saphenous veinsand allowing a maximum degree of downsizing of 40%, the followingscenario may be one example for the selection of a particular stentinner diameter to match a particular human saphenous vein. For example,given four preselected stent sizes of 3.0 mm, 3.4 mm, 3.8 mm, and 4.2mm, the following selection process may be used to match a particularhuman saphenous vein.

A first selection criteria may be set for a minimum outer vein diameternot being smaller than the stent inner diameter. Therefore, the 3.0 mmstent should be used for veins with a minimum outer diameter of between3.0 mm and 3.3 mm; the 3.4 mm stent should be used for veins with aminimum outer diameter of between 3.4 mm and 3.7 mm; the 3.8 mm stentshould be used for veins with a minimum outer diameter of between 3.8 mmand 4.1 mm; and the 4.2 mm stent should be used for veins with a minimumouter diameter of 4.2 mm and larger.

A second selection criteria may be set for a maximum degree ofdownsizing not to exceed 40%, which causes the maximum outer veindiameter for a particular stent inner diameter to be limited by thestent inner diameter that causes the maximum degree of downsizing of40%. In view of the second criteria, the 3.0 mm stent should be used forveins with a maximum outer diameter of 5.0 mm and smaller; the 3.4 mmstent should be used for veins with a maximum outer diameter of 5.7 mmand smaller; the 3.8 mm stent should be used for veins with a maximumouter diameter of 6.3 mm and smaller; and the 4.2 mm stent should beused for veins with a maximum outer diameter of 7.0 mm and smaller.

Combining the first and second selection criteria results in thefollowing categorization listed in Table IV: TABLE IV Stent SizeSelection Criteria Stent ID [mm] Mm Vein OD [mm] Max Vein OD [mm] 3.03.1-3.3 5.0 and smaller 3.4 3.4-3.7 5.7 and smaller 3.8 3.8-4.1 6.3 andsmaller 4.2 4.2 and larger 7.0 and smaller

There may be veins which do not fall in any of the size categories forvarious reasons. For example, the minimum outer vein diameter may besmaller than 3.0 mm, or the difference between minimum and maximum outervein diameter may exceed the diameter range for a particular stent size,e.g. a vein with minimum outer diameter of 3.2 mm and maximum outer veindiameter of 5.9 mm. For these veins, it should be determined whether thefollowing conditions are present: 1) either the minimum or the maximumouter diameter is limited to a very short segment of the vein, and theminimum or maximum diameter differs considerably from the outer diameterof the adjacent vein segments, or 2) the location of such a localdecrease or increase of the outer vein diameter along the harvested veinsegment allows this short segment to be removed while the resulting twovein segments can be used for the graft construction. If theseconditions apply, the segment with the minimum or maximum outer veindiameter should be removed. The stent inner diameter would then beselected for each of the two resulting vein segments according to theabove sizing criteria (see Table IV).

For veins having a minimum outer diameter smaller than 3.0 mm, and whichdo not allow the removal of the segment with this minimum diameter, alow degree of over-sizing may be permitted to allow the use of the 3.0mm stent. The degree, or percentage, of over-sizing may be expressed asbeing equal to (outer diameter vein—inner diameter stent)/(outerdiameter vein)*100, which results in a negative number. For example, ifthe minimum outer diameter of the vein is 2.8 mm, the degree ofover-sizing of the 3.0 mm stent would be limited to (negative) 7.1%.

Another study was completed to assess the dimensional variability ofhuman saphenous veins and to use the data for the mathematicaloptimization of practical aspects to guide the clinical implementationof external stenting and diameter “smoothing” or “smoothening”.“Smoothing” or “smoothening” as used herein refers to altering the outerdiameter of a section of the vein that is different (i.e., either largeror smaller) than an adjacent section with a stent so that the outerdiameter of the vein is substantially the same along its length.

In this study, in 100 consecutive patients undergoing aorto-coronarybypass grafting, the outer diameter (OD) of 118 saphenous veins wasrecorded in 2 cm incremental segments during post-harvest in situleakage-test distention using a Vernier caliper. Patient demographics(sex, race, age) and risk factors (weight, nutritional state,hypertension, diabetes and smoking) were also recorded.

The data collected was analyzed for hypothetical vessel smoothening. Asa first step, the data was mathematically categorized. Morespecifically, for each vein, the minimum and maximum values of the outerdiameter (OD_(min) and OD_(max), respectively) were identified in thedata set that was recorded along the entire length of the vein. Using astent to downsize the vein, the degree of downsizing (DS_(Sm)) requiredfor complete smoothening of the outer diameter of the vein wasdetermined for each individual vein using $\begin{matrix}{{DS}_{Sm} = {\left( \frac{{OD}_{\max} - {OD}_{\min}}{{OD}_{\max}} \right) \cdot 100.}} & (5)\end{matrix}$

From results of pre-clinical research, the limits for over-sizing(p_(o)) and under-sizing (p_(u)) of the stent relative to the vein toobtain diameter smoothening were chosen to be p_(o)=100% (noover-sizing) and p_(u)=50% of the outer diameter of the vein. With theseparameters, the admissible range for the stent diameter for eachindividual vein (Δ_(i)) can mathematically be expressed as$\begin{matrix}{\Delta_{i} = {\left\lbrack {{\max\limits_{j}\quad{\frac{p_{u}}{100}{{OD}_{i}\left( x_{j} \right)}}},{\min\limits_{j}{\frac{p_{o}}{100}{{OD}_{i}\left( x_{j} \right)}}}} \right\rbrack = \left\lbrack {{OD}_{i}^{-},{OD}_{i}^{+}} \right\rbrack}} & \left( {6a} \right) \\{or} & \quad \\\begin{matrix}{\Delta_{i} = \left\lbrack {{\max\limits_{j}{0.5{{OD}_{i}\left( x_{j} \right)}}},{\min\limits_{j}{{OD}_{i}\left( x_{j} \right)}}} \right\rbrack} \\{= \left\lbrack {{0.5{OD}_{\max,i}},{OD}_{\min,i}} \right\rbrack} \\{= {\left\lbrack {{OD}_{i}^{-},{OD}_{i}^{+}} \right\rbrack.}}\end{matrix} & \left( {6b} \right)\end{matrix}$

The upper bound for the stent diameter (OD_(i) ⁺) and lower bound forthe stent diameter (OD_(i) ⁻) for each individual vein was obtainedusingOD_(i) ⁺=OD_(min, i)   (7)andOD_(i) ⁻=0.5OD_(max,i)  (8)

Veins which did not satisfy the solution condition ofOD_(i) ⁻≦OD_(i) ⁺,   (9)because the minimum diameter of the vein was smaller than one-half ofthe maximum diameter of the vein, were excluded from the data set andfurther analysis.

The individual stent diameter solution ranges Δ_(i)=[OD_(i) ⁻, OD_(i) ⁺]with i=1, 2, . . . , 118, representing each of the 18 veins, were rankedaccording to the individual upper and lower bound. In the case thatmaximum lower bound $\left( {\max\limits_{i}{OD}_{i}^{-}} \right)$and the minimum upper bound$\left( {\min\limits_{i}{OD}_{i}^{+}} \right)$of the individual stent diameter ranges satisfied condition (10)$\begin{matrix}{{{\max\limits_{i}{OD}_{i}^{-}} \leq {\min\limits_{i}{OD}_{i}^{+}}},} & (10)\end{matrix}$the admissible range for the stent diameter accommodating the entire setof veins (D) was readily available as$\begin{matrix}{D \in {\left\lbrack {{\max\limits_{i}{OD}_{i}^{-}},{\min\limits_{i}{OD}_{i}^{+}}} \right\rbrack.}} & (11)\end{matrix}$

In the case that the condition (10) was not satisfied for the entiredata set Δ_(i) with i=1, 2, . . . , 118, the data set was divided in nsubsets, i.e., Δ_(i1, i2, . . . , in) with i₁=1, 2, . . . , m, i₂=m+1,m+2, . . . , m+k and i_(n)=m+k+1, . . . , 118 such that condition (10)was satisfied for each subset. The admissible range of the stentdiameter for each subset (D_(k)) was derived from the maximum lowerbound and minimum upper bound of the stent diameter of each subset:$\begin{matrix}{{{D_{k} \in {\left\lbrack {{\max\limits_{i}{OD}_{i}^{-}},{\min\limits_{i}{OD}_{i}^{+}}} \right\rbrack_{k}k}} = 1},2,\ldots\quad,n} & (12)\end{matrix}$

It was desired to accommodate the entire set of veins with a minimumnumber of different stent diameters (for commercial purposes), andchoose the least amount of downsizing for an individual vein. In orderto optimize for the number of different stent diameter ranges,represented by D_(k), and the required downsizing of an individual vein,or average downsizing for a subset of veins, overlap of the stentdiameter range between two vein subsets was allowed.

With the aim of a minimized number of stent diameters to accommodate theentire group of veins, two solution approaches were investigated. Thefirst solution approach included three stent diameters (D_(3,A,)D_(3,B), and D_(3,C)) and the second solution approach included twostent diameters (D_(2,A) and D_(2,B)). For both solution approaches, theassignment of individual veins to one of the stent diameters wasperformed with two alternative algorithms, such that a particular veinreceived either the smallest stent diameter for the respectiveadmissible stent diameter range of the vein, which resulted in a largerdegree of downsizing, or the largest stent diameter possible for therespective admissible stent diameter range of the vein, which resultedin a smaller degree of downsizing. The two different classificationalgorithms employed for the assignment of veins to one stent diameterare illustrated in FIGS. 44 and 45 for the first solution approachfeaturing three stent diameters (D_(3,A), D_(3,B), and D_(3,C)), withD_(3,A)<D_(3,B)<D_(3,C). Algorithm 1 (illustrated in FIG. 44) assignsthe smallest possible stent diameter to the vein, while algorithm 2(illustrated in FIG. 45) assigns the largest possible stent diameter tothe vein. For example, a vein with a lower bound of the admissible stentdiameter of OD⁻≦D_(3,A) and the upper bound of the admissible stentdiameter of OD⁺≧D_(3,B) would be assigned to the smaller stent diameter(D_(3,A)) with algorithm 1, and to the larger stent diameter (D_(3,A))with algorithm 2.

For both solution approaches and classification algorithms, thedistribution of veins among the proposed stent sizes was evaluated withrespect to the number of veins and the average degree of downsizing ineach stent size group, as well as the grand mean degree of downsizingobtained in the entire group of veins. The downsizing of veins throughstenting was also compared to the smoothing downsize degree, whichexpresses the minimum amount of downsizing to achieve completesmoothening of the vein diameter. Smoothening of a vein (i.e. alteringthe outside diameter of the vein to a substantially constant diameter)is ensured in the case where the stenting downsize degree is equal to orlarger than the smoothening downsize degree. Depending on theformulation of the sizing condition, e.g., oversizing of the stent notbeing permitted as proposed, the stenting downsize degree will alwayssatisfy this criteria for a stent assigned by employing the proposedapproach. In the case that oversizing of the stent is permitted, thesmoothening of the vein through application of a stent would need to beassessed in further detail.

The average length of the 118 harvested veins was 28.4±9.5 cm with aminimum and maximum length of 10 cm and 52 cm, respectively. The minimumand maximum outer diameter over all 118 veins was 2.1 mm and 6.5 mm withan average minimum outer diameter of 3.50±0.61 mm and average maximumouter diameter of 4.77±0.75 mm.

For the 118 harvested veins, the average downsizing required forcomplete smoothening of the vein diameter according to equation (5)above was 26.0±11.1%, with a minimum of 0% and a maximum of 57.2%. Thedistribution of veins in incremental classes according to thesmoothening downsize degree (Ds_(Sm)) i.e., 0%≦Ds_(Sm)<10%,10%≦DS_(Sm)<20%, etc., and the proportion of the total number of veinsis summarized in Table V below. TABLE V Distribution of Harvested VeinsAccording to Classified Smoothening Downsize Degree (DS_(Sm)) Downsizing[%] No of veins Proportion [%]  0 ≦ DS_(Sm) < 10 6 5.1 10 ≦ DS_(Sm) < 2030 25.4 20 ≦ DS_(Sm) < 30 45 38.1 30 ≦ DS_(Sm) < 40 19 16.1 40 ≦ DS_(Sm)< 50 17 14.4 DS_(Sm) ≧ 50 1 0.8

The largest proportion of veins, 38.1%, needed moderate downsizingbetween 20% and 30% to achieve diameter smoothening, whereas only onevein exceeded a downsize requirement of 50%. The number of veins appearsto be distributed normally among the downsizing classes, with a slightskew towards the lower downsizing degrees. With the proposed limits ofoversizing and downsizing of a vein, i.e., no oversizing and maximum 50%downsizing, and the related upper and lower bounds of the stent diameterformulated in equations (7) and (8) above, 117 of the 118 veins, or99.2%, satisfied the condition formulated in equation (9) above ofOD_(i) ⁻≦OD_(i) ⁺. More specifically, for 115 veins, the lower bound ofthe individual admissible stent diameter range was less than the upperbound of the individual admissible stent diameter range (OD_(i) ⁻<OD_(i)⁺), and for 2 veins, there was a single value for the individualadmissible stent diameter, as OD_(i) ⁻=OD_(i) ⁺.

For one vein, the lower stent diameter bound exceeded the upper bound.This vein did not satisfy equation (9) above, and was excluded from thefurther analysis. This vein required more than 50%, specifically 57.2%,downsizing for diameter smoothening, which was outside of the initialpermissible percentage of undersizing the stent (p_(u)). The individualadmissible stent diameter ranges (Δ_(i)) for the entire set of 118 veinsbased on the conditions of no oversizing and downsizing up to maximum of50% are illustrated in FIG. 46. The negative stent diameter range shownin FIG. 46 indicates the one vein that did not qualify, due to the lowerstent diameter bound (OD_(i) ⁻) exceeding the upper stent diameter bound(OD_(i) ⁺), and was disregarded from further analysis. In FIG. 46, thestent diameter ranges are ranked according to the upper stent diameterbound (primary ranking parameter) and the lower stent diameter bound(secondary ranking parameter).

The lower bound for the stent diameter (OD_(i) ⁻) ranged between 1.65 mmand 3.25 mm with a mean of 2.39±0.38 mm. The upper bound for the stentdiameter (OD_(i) ⁺) ranged between 2.10 mm and 5.50 mm with a mean valueof 3.50±0.61 mm.

With a maximum value of the lower bound of the stent diameter$\left( {\max\limits_{i}{OD}_{i}^{-}} \right)\quad{of}$3.25 mm and the minimum upper bound of the stent diameter$\left( {\min\limits_{i}{OD}_{i}^{+}} \right)$of 2.10 mm, the condition for solution of the admissible stent diameterrange, i.e.${{\max\limits_{i}{OD}_{i}^{-}} \leq {\min\limits_{i}{OD}_{i}^{+}}},$was not satisfied. Consequently, a single stent diameter, or stentdiameter range, accommodating all 117 veins could not be identified withthe proposed downsizing and oversizing conditions.

To accommodate the individual stent diameter ranges presented in FIG.46, stent diameters of D_(3,A)=3.0 mm, D_(3,B)=3.5 mm and D_(3,C)=4.0 mmwere proposed for the 3-stent-solution, whereas diameters of D_(2,A)=2.9mm and D_(2,B)=3.3 mm were proposed for the 2-stent-solution. For eithersolution approach, the classification of individual veins according tothe stent diameters suggested was performed in two alternative fashionsseeking for the smallest stent size and largest stent size for eachvein. As discussed above, the underlying classification algorithms areillustrated in FIGS. 44 and 45 for the 3-stent solution approach.

The solution approach featuring three stent diameters D_(3,A)=3.0 mm,D_(3,B)=3.5 mm and D_(3,C)=4.0 mm resulted in 14 veins not beingaccommodated in any of the three stent sizes, independent of theclassification algorithm used. Thirteen veins exhibited an upper stentdiameter bound of less then 3.0 mm, and therefore required a stentdiameter smaller than 3.0 mm to satisfy the proposed downsizing andoversizing conditions. One vein had a stent diameter range between 3.15mm and 3.4 mm, so none of the pre-selected stent diameters couldaccommodate that vein. The classification of the remaining 102 veinswith algorithm 1 (minimizing the stent diameter and maximizing thedownsize degree) assigned 95 veins to the stent diameter of 3.0 mm, 7veins to the stent diameter of 3.5 mm, and no vein was assigned to the4.0 mm stent diameter. The classification of the 102 veins withalgorithm 2 (maximizing the stent diameter and minimizing the downsizedegree) resulted in the following distribution of the veins: 35 veinsfor 3.0 mm stent diameter, 43 veins for 3.5 mm stent, and 24 veins for4.0 mm stent diameter. The vein classification and resulting downsizingin the different diameter groups of the 3-stent-diameter approach issummarized in Table VI below. TABLE VI Distribution of veins andresulting downsizing for 3-stent- diameter solution approachClassification Excluded Stent Diameter [mm] Option Parameter veins 3.03.5 4.0 All Minimum N 15 95 7 0 102 Stent Diameter Mean DS_(St) [%] —35.8 ± 8.6  44.5 ± 1.6  — 36.4 ± 8.6  Maximum Min DS_(St) [%] — 9.1 42.6— 9.1 Downsizing Max DS_(St) [%] — 50.0 46.2 — 50.0 (Algorithm 1) MeanDS_(Sm) [%] — 24.0 ± 9.8  29.2 ± 11.2 — 24.3 ± 10.0 DS_(St)/DS_(Sm) —1.68 ± 0.71 1.79 ± 0.87 — 1.69 ± 0.72 Maximum N 15 35 43 24 102 StentDiameter Mean DS_(St) [%] — 32.3 ± 10.4 26.7 ± 9.0  24.2 ± 8.1 28.1 ±9.8  Minimum Min DS_(St) [%] — 9.1 7.9 7.0 7.0 Downsizing Max DS_(St)[%] — 50.0 46.2 38.5 50.0 (Algorithm 2) Mean DS_(Sm) [%] — 30.1 ± 9.8 23.3 ± 9.2  17.8 ± 6.4 24.3 ± 10.0 DS_(St)/DS_(Sm) — 1.12 ± 0.34 1.19 ±0.23  1.41 ± 0.48 1.22 ± 0.35DS_(St) = stenting downsizing degreeDS_(Sm) = smoothing downsize degree

The smoothening downsize degree (DS_(Sm)) refers to the downsizingrequired to completely smoothen a vein, i.e., alter the maximum vein ODto the minimum vein OD. The stenting downsizing degree (DS_(St)) refersto the downsizing a vein experiences, i.e., amount the OD of a veinchanges, by having a stent on the outside of the vein.

The classification towards least stenting downsizing degree resulted ina more even distribution of the veins among the three stent diametersproposed, whereas for maximum downsizing, the majority of veins (91.1%)were assigned to the smallest stent diameter, while the largest stentdiameter was left unpopulated.

The classification of the 117 veins employing the two-stent solutionapproach with stent diameters of D_(2,A)=2.9 mm and D_(2,B)=3.3 mmexcluded 11 veins due to a upper stent diameter bound of less then 2.9mm, again independently of the algorithm used. From the remaining 106veins, 93 and 13 veins were assigned to the 2.9 mm and 3.3 mm stent,respectively, when the minimum stent diameter associated with themaximum downsize degree was targeted for each vein (classificationalgorithm 1). Aiming at the least stenting downsizing degree byassigning the largest possible stent to each vein, the 2.9 mm stent wasassigned to 30 veins and the 3.3 mm stent to 76 veins. The mean stentingdownsize degree for the two options of maximum and minimum downsizingwas 36.7±8.2% and 31.2±9.7%, respectively, for the 2.9 mm stent and46.6±1.8% and 33.7±8.4%, respectively, for the 3.3 mm stent. The entiredata set is summarized in Table VII below. TABLE VII Distribution ofveins and resulting downsizing for 2-stent- diameter solution approach.Classification Excluded Stent Diameter [mm] Option Parameter Veins 2.93.3 All Minimum Stent N 11 93   13   106   Diameter Mean DS_(St) [%] —36.7 ± 8.2 46.6 ± 1.8 38.0 ± 8.4  Maximum Min DS_(St) [%] — 12.1 44.112.1 Downsizing Max DS_(St) [%] 36.7 ± 8.2 46.6 ± 1.8 38.0 ± 8.4 (Algorithm 1) Mean DS_(Sm) [%] 23.3 ± 9.5  33.3 ± 10.5 24.5 ± 10.1DS_(St) /DS_(Sm) —  1.79 ± 0.78  1.61 ± 0.77 1.77 ± 0.78 Maximum Stent N11 30   76   106   Diameter Mean DS_(St) [%] — 31.2 ± 9.7 33.7 ± 8.433.0 ± 8.8  Minimum Min DS_(St) [%] — 12.1 13.2 12.1 Downsizing MaxDS_(St) [%] — 49.1 49.2 49.2 (Algorithm 2) Mean DS_(Sm) [%]  28.4 ± 10.223.0 ± 9.7 24.5 ± 10.1 DS_(St)/DS_(Sm)  1.12 ± 0.34  1.19 ± 0.23 1.41 ±0.48DS_(St) = stenting downsizing degreeDS_(Sm) = smoothing downsize degree

To further assess and rank the various proposed solution approaches (3stent versus 2 stent diameters, and maximum downsizing versus minimumdownsizing), the downsizing of the veins through the assigned stentdiameter was compared to the smoothening downsize degree. The ratio ofstenting downsize degree to smoothening downsize degree,DS_(St)/DS_(Sm), was required to be equal or larger than 1 to achievecomplete diameter smoothening. Considering the grand mean of the ratioDS_(St)/DS_(Sm) for all veins which received a stent, the smallestdegree of downsizing was obtained with the 3-stent solution approach incombination with the classification algorithm 2 opting for leastdownsizing (DS_(St)/DS_(Sm)=1.22±0.35), followed by the 2-stent approachwith the same classification algorithm (DS_(St)/DS_(Sm)=1.41±0.48), andthe 3-stent and 2-stent approach with the alternative classificationalgorithm (DS_(St)/DS_(Sm)=1.69±0.72 and 1.77±0.78, respectively).

For the condition that oversizing of the stent is not permitted, thesolution approaches proposed ensured that the upper bound of theadmissible stent diameter for each vein did not become smaller thanminimum vein diameter for any one vein, and hence resulted in completesmoothening. This was confirmed for both solution approaches andclassification algorithms in that the minimum of the ratio of stentingdownsize degree to smoothening downsize degree, DS_(St)/DS_(Sm), wasequal to 1. The values of the DS_(St)/DS_(Sm) ratio for different stentdiameter groups, along with the group mean values of the smootheningdownsize degree, are given in Table VI for the 3-stent solution andTable VII for the 2-stent solution. The 3-stent solution with leastdownsize classification resulted in the closest match between stentingdownsize degree and smoothening downsize degree for each stent diameterwhereas the 2-stent solution 1 with maximum downsize classificationreturned the largest differences between stenting downsize degree andsmoothening downsize degree (following the same order than for the grandmeans of the DS_(St)/DS_(Sm) ratio).

Hypothetical application of external stents equivalent in size to thecorresponding minimum outer diameter of each vein resulted in a meanmaximum downsizing of 26.0±1.0% (range 0 to 57.1%). Whereas thissolution is an unpractical one, since 86 differently sized stents wouldbe required to accommodate the 118 veins accepting a tolerance of 0.1mm, the fact that 117 veins were maximally downsized by 50% or lesssuggests a phenomenon intrinsic to the lengths of human saphenous veinharvested, namely that the maximum outer diameter rarely exceeds theminimum outer diameter by more than 50%. FIG. 47 shows the relationshipbetween the OD_(min) and OD_(max) for each vein. Also shown in FIG. 47are lines representing an amount of downsizing ranging from 10% to 50%.By increasing the amount of downsizing, an increased number of veins maybe accommodated, thereby confirming again the intrinsic constraint ofthe relationship between these two parameters. The relationship betweenthe maximum downsizing limit and the proportion of veins therebyaccommodated is shown in FIG. 48. The falloff of veins is dramatic withan increased setting of the downsizing limit, and indicates that adownsizing limit of not less than 50% should be used. Therefore, withoutlimiting the number of available stent sizes and by calculating theproportion of veins with an upper tolerance interval not exceeding 50%,simply applying a stent with a fully distended inner diameter equal tothe OD_(min) of the vein, both the ‘zero-oversizing’ and ‘50% maximumdownsizing’ rules were seen to be met in 99% of veins with 95%confidence.

Hypothetical application of an external stent with an internal diameterequivalent to the overall minimum outer diameter observed in all veins,namely 2.1 mm, may avoid the requirement for an excessive number ofstent solutions and also any upsizing which would occur at a point wherethe vessel outer diameter would be smaller than the inner diameter ofthe applied stent. However, excessive downsizing may result, which maynot be desirable.

In order to optimize the number and sizing of stents, the technique ofrecursive partitioning was applied to the vein data. This methoddeployed by JMP, a statistical software package (version 6.0.3, Cary,N.C.), recursively partitions, in this case veins, based on ahypothetical relationship between the minimum OD and maximum OD/minimumOD ratio for each vein, thereby creating a tree of optimal OD minimacut-points for prediction of maxima/minima ratios within the cohort ofveins being studied. It does this by an iterative process where allpossible cut-points (and thus groupings) are examined. The process ismanually advanced until a desired fit (e.g. a plateau in the R2 values)is seen to be reached with the minimum number of partitions. The resultof these partitions suggested stent sizes of 3.5 mm, 2.7 mm, 3.9 mm and3.3 mm in that order. Hypothetical application of these stent sizes tosimulate single, double, triple and quadruple solutions revealedacceptable downsizing (<50%) but with a moderate degree of oversizing inthe case of the deployment of a single 3.5 mm stent. However,distribution of both the 3.5 mm and 2.7 mm stents based on the OD_(min)for each vein confirmed success in 100% of veins with no oversizing andnot more than 50% downsizing. The addition of the third 3.9 mm andfourth 3.3 mm stents into the solution did not appear to affect theextent of downsizing at all despite a fairly even distribution of thecohort of veins across the four stent sizes.

Of course, the above identified stent sizes are provided as examplesonly. For example, an alternative selection of stent sizes may includediameters of 3.6 mm, 2.8 mm, 4.0 mm, and 3.4 mm, etc. Theabove-identified stent sizes are not intended to be limiting in any way.

For ease of use by surgeons, the selection criteria for stent size maybe utilized in a selection tool such as a “dial disc” or a look-up tablewhere, in the simplest case, the surgeon has to select the minimum andmaximum vein outer diameters on two independent scales and the “dialdisc” returns the appropriate stent size, or a recommendation for anadjustment procedure should the vein diameters do not fall within onestent size group.

In a more sophisticated version of the “dial disc”, the surgeon would berequired to enter or select the minimum and maximum vein diameters andone or two additional selection criteria, such as desired maximumdownsizing, in order to obtain a recommendation for the suitable stentsize.

It is also contemplated that such a selection tool may be in the form ofa computer program in which the surgeon enters the minimum and maximumvein diameters, and, optionally, the desired maximum downsizing, and theprogram calculates and displays the most suitable stent size based onthe values inputted.

For example, the surgeon (or his/her designee) may enter the requireddata into a computer (e.g. a PC, PDA, etc.) via a user input (e.g., akeyboard or mouse selecting entries on a graphical user interface). Thesoftware program resides on a computer readable medium (e.g., a harddrive, RAM, or an insertable/removable memory like a CD, DVD, or floppydisk). The software program comprises computer executable code forperforming the calculating method (which may be in accordance with anyembodiment described herein or any variant thereof), and displays theresults to the surgeon and/or his designee. The program may reside on alocal computer at which the surgeon or his/her designee works. Likewise,the software program may be network based, such that the programexecutes at a remote location, with the inputs and results beingtransmitted to and from the local computer at which the surgeon orhis/her designee works. The display may be accomplished via a graphicaluser interface (e.g., a display screen), or by printing out the results.Thus, the scope of the invention includes the software programcomprising computer executable instructions for performing such methods.

Overview of Examples

For the repair of diseased vessels using autologous tissue, thesaphenous vein graft is the surgeon's main option following the use ofarterial tissue e.g., internal mammary artery (IMA) and radial artery.Unfortunately, performance of saphenous vein grafts is substantiallyless than arterial tissue, given the thin-walled vein architecture andtendency to undergo excessive remodeling following exposure to arterialpressure. This excessive remodeling is thought to be mainly due totissue damage and ensuing unchecked cellular responses as the venoustissue experiences dimensions, pressures, and pulses not seen in itsnative venous environment. Ultimately, such a response can lead to graftstenosis and occlusion.

It has been shown shown that an external stent placed over the saphenousvein at implantation prevents excessive vein dilatation upon exposure toarterial blood flow. A markedly reduced remodeling response has alsobeen seen in these externally stented veins. In the previous studies,the external saphenous vein stents had minimal or no natural radialcompliance incorporated into the stent design. It was hypothesized thata saphenous vein surrounded by a stent engineered to have more naturalradial compliance i.e., one close to that of native arterial tissue,will undergo an even more favorable remodeling response. The latter willbe reflected by histological evidence of a more arterial-likearchitecture following exposure the arterial environment. Ultimately,the goal was for this favorable remodeling to translate to an improvedsaphenous vein graft patency compared to non-externally-stentedcontrols.

As shown in the Examples below, external mechanical reinforcement ofvein grafts with size-matching stents prevented intimal hyperplasticresponse and maintained the lumenal dimension of the grafts ensuringconsistent flow condition in the grafts over time and compared to thehost arteries. Example 1 shows that using a non-compliant braided wirestents to support grafted femoral vein segments results in 100% patencyat 6 weeks and 75% patency at 12 weeks, while unsupported femoral veingrafts had 100% patency, but only 50% patents at 12 weeks.

As shown in the Examples below, downsizing of vein grafts with externalstents having a smaller diameter than the grafted vein segment,considerably limited the intimal and adventitial hyperplastic response,and prevented endothelial damage, compared to vein grafts with oversizedexternal stent support (see Example 2). Thus, downsizing of vein graftswith external stents was found to be superior to oversized externalsupport, and as good as external mechanical support matching the size ofthe vein in venous circulation (see, Examples 1 and 2).

As shown in the Examples below, a saphenous vein graft supported by anexternal stent with natural radial compliance resulted in remodelinginto a histological architecture consistent with arterial tissue.Braided and knitted stents with low or high radial compliance were usedto show the effect of radial compliance on remodeling and graft patency,and were further used for comparing and contrasting the healing responseof the stents produced by different manufacturing methods (See Example4). With increasing time after implantation, braided stents showed anincrease in lumen diameter, while knitted stents did not show anincrease in lumen diameter, indicating that the knitted stents providedgreater size stability for the vein graft over time. (See Example 4).

In previous studies, external saphenous vein stents were tested only inthe peripheral vasculature. However, a main intended application is thecoronary vasculature. As shown in the Examples below, a canine modelconfirmed the hypothesis that a canine femoral vein surrounded by astent engineered to have a natural radial compliance (i.e., one close tothat of native arterial tissue) will undergo a favorable remodelingresponse when implanted in the coronary vasculature, and morearterial-like architecture would be found in the vein grafts uponexplantation (see Examples 3.A. and 3.B. In a baboon model, thefeasibility of using a saphenous vein surrounded by knitted compliantstent for grafting in an aorto-coronary (CABG) position wasdemonstrated, where stented saphenous vein CABG grafts had a highpatency rate, and showed a favorable remodeling response (see Example5).

As noted above, the external saphenous vein stents in earlier studieshad minimal or no radial compliance incorporated into the stent design.In later phases, two types of stent designs were studied along with acrimping feature intended to provide increased radial (circumferential)compliance (see Example 4).

EXAMPLE 1 Effect of External Reinforcement of Vein Grafts on theRemodeling of the Veins Transposed into Arterial Circulation

Briefly, a baboon bilateral femoral artery vein graft model was used ina two-factor study to assess the effects of stented and non-stented veingrafts, and implant duration (6 weeks and 12 weeks) in a total of eight(8) baboons.

Protocols

The non-compliant stented vein grafts used in this study were segmentsof superficial femoral vein stented with Nitinol (NiTi)-wire tubularbraid stents with an OD of 5 mm, assumed to be non-compliant in a radialdirection such that no change in diameter due to changing blood pressurewould be permitted. The majority of stents used for implantations were36 carrier (wire) Nitinol stents with 0.025 mm wire thickness, with anexpected compliance of between about 1.6 to 2.3%/100 mmHg. Oneimplantation for the 6 week implant duration included one pilotimplantation with a stent having 72 carriers (wires) and 0.050 mm wirethickness.

Adult Chacma baboons (5 female, 3 male, “senescent Chacma Baboon model”)were anesthetized and intubated. The femoral artery and vein wereexposed through a longitudinal incision of approximately 15 cm. Afteradministration of heparin (250 U/kg), the superficial femoral vein (SFV)was clamped distal to the origin of the deep femoral vein and as fardistally as possible before disappearing in the adductor channel. Insome baboons, prior to clamping, the in situ vein diameter was estimatedby approximation using the jaws of a vernier caliper. The adjacentsuperficial femoral artery (SFA) was equally clamped after the origin ofthe deep femoral artery and distally at the same level as the SFV. Afterexcision of a comparable segment of the SFA, the reversed SFV wasanastomosed to the distal segment of the artery in an end-to-endfashion, using a continuous running suture of 7-0 Prolene. In thecontrol group, the proximal end-to-end anastomosis was completed in asimilar fashion. In the experimental group, a piece of Nitinol stent ofthe same length as the vein segment was cut off and the vein was gentlypulled through the stent after the distal end-to-end anastomosis hadbeen completed. The proximal end-to-end anastomosis was completed,similarly using a running suture of 7/0 Prolene. In the experimentalgroup, the Nitinol stent was secured to the arterial adventitia oneither end over the anastomosis, using a superficial single suture of7/0 Prolene. After removal of the arterial clamps and restoration of theblood flow, outside diameters of the mid-section of the control veingrafts were measured using a vernier caliper.

Macrophotographs of the implanted graphs were taken and, after ensuringadequate haemostasis, the incisions were closed in two layers (2/0Vicryl sutures for subcutaneous layer and 2/0 Nylon sutures for skin).

Explantation:

At 6 or 12 weeks after implantation, the vein grafts were evaluated forpatency and explanted. After sedation, intubation, and generalanaesthesia, the skin was incised on both sides from inguinal towardscaudal direction according to the length of the implanted graft. Thesuperficial femoral artery was prepared at both ends proximal and distalof the graft (±1.5.cm from the anastomosis) and ligatures were placedbut not yet tied. Subsequently the animal was fully heparinized (250IU/kg) and sacrificed by a bolus injection of a KCI (40 mmol) andcessation of spontaneous cardiac activity was verified on the ECGmonitor before ventilation was stopped. The femoral arteries werecannulated proximally and distally to the graft within theabove-mentioned ligatures. The prostheses were each rinsed in situ with100 ml PBS and fixed with 100 ml 10% FA under pressure. Finally thegrafts were excised en-bloc with approximately one centimeter ofsurrounding tissue. The proximal side was marked with a liga-clip beforethe explanted grafts were placed in a container filled with PBS/FA forfurther preparation.

Sample Processing:

The first macroscopic pictures of the explanted sample were taken afterexcess tissue was trimmed from the vein graft . Subsequently, the graftswere carefully opened longitudinally from each end towards the mid-graftregion (in order to positively identify the middle of the graft). Onceidentified, and before the longitudinal incision was completed, an 8 mmsection (denoted section 3) was removed as an intact graft ring. For the6-week implant duration study, this sample was sub-sectioned into a 3 mmring-section for later SEM analysis, and a 5 mm section for lightmicroscopy. For the 12-week implant duration study, a 1 cm ring sectionwas taken for light microscopy whereas later SEM analyses used theadjacent graft segments. Additional higher magnification images of thering-section (section 3) were taken by conventional macrophotography andby using a stereo microscope, and images and data were captured on aFilemaker database.

For macroscopical image analysis, the ring-section was placed onto aruler and straight vertical pictures were taken and imported into theQ-Win Image analysis Unit. The following parameters were calculated:cross sectional area; theoretical mean lumenal diameter (calculated fromperimeter measurements; but due to the irregularity of the perimeter asa result of IH, this theoretically calculated diameter always representsan overestimation of the true mean diameter); and minimum/maximumlumenal diameter (directly measured; because of the overestimateobtained by calculating the mean diameter from the perimeter, it ispossible that the measured maximum diameter is smaller than thecalculated mean diameter.)

For light microscopy, vein grafts were fixed in 10% formalin (in PBS, pH7,4) for 24 hours, then transferred into 70% ethanol before furtherprocessing as described below.

Embedded non-stented vein grafts (“controls”) were prepared byprocessing through increasing concentrations of alcohol (70% to 100%),and cleared using 2,2,4 trimethylpentane (3 changes) and subsequentlyinfiltrated and embedded in paraffin wax. Sectioning was done on aMICROM H360 heavy duty microtome(3 micron sections). Slides were stainedfor Haematoxylon and Eosin, Victoria Blue and immunocytochemically forCD 31 and Actin.

Embedded stented vein grafts were prepared as follows. Light microscopywas entirely done on the basis of resin embedding. For the 6-weekimplant duration study, half of the ring-sections for light microscopywere resin embedded whereas the other half underwent the manual removalof the stent wires under a stereo microscope by a fine-pointed forcepsprior to being wax embedded like the (non-stented) controls. Samples forresin embedding were hand-processed through 100% alcohol at 4° C.,infiltrated with 2 changes of Technovit 8100 infiltration solution on asample roller (overnight 4° C.), and subsequently embedded in T8100(under vacuum, 0° C., 3 hours) and sectioned on an Isomet Precision Saw2000 (Buehler), resulting in thick (100 micron) sections. The sectionswere then ground down using a Metaserv 2000 grinder using 2 differentgrades of sand paper in order to obtain reasonably thin sections. Theseslides were then stained either Haematoxylin and Eosin (“HE”) as well asa Masson's trichrome stain Resin was not removed and slides were mountedusing an aqueous mounting medium. Wax sections were routinely stainedHE, Azan, Movat and Victoria Blue.

Image Analysis

A Leitz DM RB microscope with an attached Leica DC200 digital camera wasused to visualize and capture color micrographs of the vein grafts.Macropictures were directly imported to a computer equipped for imageanalysis. Image analysis was performed using Leica Q-Win 500 software.Briefly, after capturing images of Azan/Movat stained sections, 10-25measuring fields of each sample capturing the entire specimen were usedfor the analysis, depending on the size of the particular vein graft.Interactive measurements were used to highlight the area of interest bydetection-filtering of the color images (achieved by threshholding). Theparameters measured were: lumenal cross sectional area and calculatedmean lumenal diameter; lumenal minimum and maximum diameter; crosssectional area of intima (intimal hyperplasia, IH) tissue; thickness ofIH tissue layer (minimum, maximum, mean); cross sectional area of medialtissue; thickness of media (minimum, maximum, mean); proportion of mediatissue consisting of SMCs; cross sectional area of adventitial(advential hyperplasia, AH) tissue; thickness of AH tissue layer(minimum, maximum, mean)

The following histology sections were used: complete ring section of midgraft section of each graft; longitudinal sections of proximal anddistal anastomotic region of each graft; sections stained with Movatstains, Azan stains, Victoria Blue stains, and HE stains.

Fields were counted as described below. Layers were defined according tothe following criteria: (1) Intimal Hyperplasia (IH) Tissue: thecombined use of both the presence of often clearly visible stretches ofinternal elastic membrane and the presence of light-blue groundsubstance within the IH tissue on modified Movat stains made itrelatively easy to delineate manually the boundaries of the IH tissue.(2) Media: On the modified Movat stains, the smooth muscle cells appeardark brown, while collagen is red. A smooth line was manually drawnalongside the outside perimeter of the media to delineate it from theadventitia. (3) Adventitial Hyperplasia: This layer was easilydelineated as the area between the line described above and the areawhere the stent had been (evidenced by the gaps in the tissue where thestent wires had been removed.)

Mid-graft sections: (1) The lumenal cross sectional area was measuredfrom a 0.5× image taken from a mid-graft ring section with a macro lens.Manual calibration was used to accommodate the 0.5× objective in theimage analysis program. (2) Intimal Hyperplasia and Media assessmentwere obtained from the image analysis of composite pictures taken fromthe entire circumference of the graft at 10× magnification (in average10-25 fields—depending on the size of the graft). (3) AdventitialHyperplasia was assessed from the image analysis of composite picturestaken from the entire circumference of the graft at 5× magnification.

Anastomotic sections: (1) Anastomotic Intimal Hyperplasia assessment wasobtained from the image analysis of composite pictures taken from theentire length of the section at 1.6× magnification. (2) Media wasassessed from the image analysis of composite pictures taken from theentire length of the section at 10× magnification. (3) AdventitialHyperplasia was assessed from the image analysis of composite picturestaken from the entire length of the section at 5× magnification.

Statistical Analysis of Midgraft Data and Anastomotic Data:

The microscopical image analysis for midgraft sections was consistentfor all grafts such that data was always related to full cross sections,i.e. the entire circumference of midgraft ring sections. Longitudinalanastomotic sections may, however, have varied in length available foranalysis for different grafts. In order to ensure consistency in theanalysis of the anastomotic sections, anastomotic data for a length of 4mm from the anastomoses towards the midgraft region was included in thestatistical analysis. Any data obtained from longitudinal sectionsbeyond 4 mm distance from the anastomoses was discarded from thestatistical analysis, however, has been retained in the study records.

All data of cross-sectional area (e.g. IH area, AU area, media area) aresums of individual measurements of microscopical composite images. Alldata of thicknesses (e.g. IH thickness, AH thickness, media thickness)and ratios (e.g. proportion of smooth muscle cells in media layer) aremean values of individual measurements of microscopical compositeimages. The microscopic image analysis includes the entirecross-sectional circumference of graft wall for annular midgraftsections but only 4 mm graft wall for longitudinal anastomotic sections.Hence cumulative data based on cross-sectional area measurements isdeemed as not suitable for comparison of midgraft sections andanastomotic sections. Any comparison between midgraft sections andanastomotic sections need to be based on thickness data and ratio data.

Graft Patency

In both study groups, all 6 week implants were patent (open). After 12weeks, 50% of the non-stented vein grafts (“controls”) were patent and75% of the stented vein grafts were patent, i.e., 25% of the stentedvein grafts were occluded.

At 6 weeks, macroscopic assessment of lumenal dimensions showed that, inall baboons, there was a distinct difference between control grafts andNitinol-stented grafts. While stented vein grafts had a delicate thinvein wall with an even lumen, controls (non-stented) showed marked wallthickening and irregular narrowing of the lumen. However, the innerdiameter did not significantly differ between the two groups. Equally,the cross sectional area was not significantly different. In view of themarkedly thickened wall and the dilated outside diameter of controlgrafts at implantation, intimal thickening partly compensated for theinitial dilation after 6 weeks.

At 12 weeks, macroscopic assessment of lumenal dimensions showed thatthere were only two patent control (non-stented) vein grafts, both ofwhich appeared grossly dilated. This dilatation led to an increase of163% in cross sectional area. In contrast, the stented vein graftsunderwent a mild decrease of 21% in the lumenal cross-sectional area.The wall thickness seemed less pronounced after 12 weeks than after 6weeks.

Data from Midgraft Region and Anastomotic Regions

Measurements of Midgraft region: The lumenal cross-sectional area was16.9±5.2 mm2 (42 days) and 44.3±3.8 mm2 (84 days) in the non-stentedgroup and 19.6±2.3 mm2 (42 days) and 16.3±1.3 mm2 (84 days) in thestented group. Intimal hyperplasia (IH): The mean IH thickness was110.5±6.8 μm (42 days) and 67.3±15.0 μm (84 days) in non-stented veingrafts and 5.1±5.1˜Lm (42 days) and 12.2±12.2 μm (84 days) in stentedvein grafts. Cross-sectional IH area was 1.6±0.2 mm2 (42 days) and1.7±0.4 mm2 (84 days) in the non-stented vein grafts and 0.1±0.1 mm2 (42days) and 0.2±0.2 mm2 (84 days) in stented vein grafts. The differenceof IH area between the non-stented group and the stented group wasstatistically significant at 42 days (p=0.001) and at 84 days (p=0.02).Media: The mean thickness of the media layer was 197.6±11.3 μm (42 days)and 85.1±7.2 μm (84 days) in non-stented vein grafts and 102.6±13.8 μm(42 days) and 44.4±6.2 μm (84 days) in stented vein grafts. Thecross-sectional media area was 2.9±0.1 mm2 (42 days) and 2.2±0.1 mm2 (84days) in the non-stented group compared to 1.5±0.2 mm2 (42 days) and0.6±0.1 mm2 (84 days) in the stented group. The difference in crosssectional media area between the two groups was statisticallysignificant at 42 days (p=0.002) and at 84 days (p=0.004). Adventitialhypeiplasia (AH): The mean thickness of adventitial hyperplasia was313.1±21.6 μm (42 days) and 137.0±5.8 μm (84 days) in non-stented veingrafts and 343.5±79.4 μm (42 days) and 177.4±49.5 μm (84 days) instented vein grafts. The cross-sectional AH area was 4.5±0.5 mm2 (42days) and 3.1±0.1 mm2 (84 days) in the non-stented group compared to4.9±1.1 mm2 (42 days) and 2.3±0.6 mm2 (84 days) in the stented group.The difference of AH area between of non-stented grafts and stentedgrafts was statistically not significant at 42 days (p=0.8) and at 84days (p=0.4).

Proximal anastomosis: Mean pannus thickness was 192.6±55.5 μm (42 days)and 393.6±279.1 μm (84 days) in non-stented vein grafts and 122.0 μm (42days) and 260.34±151.9 μm (84 days) in stented vein grafts. Mean mediathickness was 183.6±14.3 μm (42 days) and 118.9±52.4 μm (84 days) in thenon-stented group compared to 170.2 μm (42 days) and 273.6±153.2 μm (84days) in the stented group. Mean AH thickness was 433.1±36.2 μm (42days) and 256.4±63.0 μm (84 days) in the non-stented group compared to594.8 μm (42 days) and 597.8±159.6 μm (84 days) in the stented group.

Distal anastomosis: Mean pannus thickness was 309.7±74.3 μm (42 days)and 64.5±50.8 μm (84 days) in non-stented vein grafts and 401.6 μm (42days) and 49.8±19.2 μm (84 days) in stented vein grafts. Mean mediathickness was 224.9±23.7 μm (42 days) and 80.8±5.2 μm (84 days) in thenon-stented group compared to 159.7 μm (42 days) and 63.8±13.7 μm (84days) in the stented group. Mean AH thickness was 383.5±23.4 μm (42days) and 250.7±28.2 μm (84 days) in the non-stented group compared to318.5 μm (42 days) and 245.2±10.7 μm (84 days) in the stented group.

Summary

In non-stented grafts, exposure to arterial pressure led to a nearly 70%increase in lumenal diameter, intimal hyperplasia was pronounced after 6weeks and over-corrected the 70% distension to below the in-situdimensions, and between weeks 6 and 12, the dominant event was excessivedilation (±170%), with stagnant intimal hyperplasia. Since maximaladventitial distension was the starting point, vein graft dilation mustbe a result of failed remodeling. Successful remodeling would havere-established the flow-velocity of the artery, to which the vein wasanastomosed, by narrowing the lumen. The loosely arranged andnon-oriented media became mildly compacted and moderately aligned.

In stented vein grafts (using non-compliant stents), there waspractically no intimal hyperplasia either after 6 weeks or after 12weeks. The 5 mm stent used in the study matched the in-situ size of thevein in this model (with very mild down-sizing). The lumenal dimensionsof stented grafts remained fairly constant over 12 weeks. The mildreduction in lumenal cross-sectional area after 12 weeks was rather dueto adventitial tissue development than intimal hyperplasia. The looselyarranged media of the vein re-modeled within 6 weeksdays to form acompact, circularly aligned media. The non-compliant stent protectionappeared to be responsible for the semi-atrophic development of a thinmedia and the lack of growth between 6 weeks and 12 weeks. The exposureof non-stented grafts to arterial pressure led to nearly 70% increase inlumenal diameter. Intimal hyperplasia was pronounced after 6 weeks daysand over-corrected the 70 distension to below the in-situ veindimensions. Between 6 and 12 weeks, the dominant event was excessivedilation (±170%), with stagnant intimal hyperplasia. The looselyarranged and non-oriented media became mildly compacted and moderatelyaligned. Since maximal adventitial distension was the starting point, itis proposed that vein graft dilation was a result of failed remodeling.Successful remodeling would have re-established the flow-velocity of theartery, to which the vein was anastomosed, by narrowing the lumen.

EXAMPLE 2 Stent Size: Effect of Down-Sizing and Oversizing of ExternalReinforcement of Vein Grants on Remodeling of Veins Transposed IntoCirculation

A baboon bilateral femoral artery vein graft model was used in atwo-factor study to assess the effect of stent size in stented femoralvein grafts at two implant durations. A “senescent Chacma Baboon” model,with the superficial femoral vein used as end-to-end interposition graftin the superficial femoral artery, using protocols similar to thosedescribed in Example 1, above, was used to implement a study designwherein one experimental group of four (4) baboons was implanted withvein grafts externally stented with non-compliant Nitinol braided stentsof 6.6 mm internal diameter (Group 1, “6.6 mm stented vein grafts”), andone experimental group of four (4) baboons was implanted with veingrafts externally stented with non-compliant Nitinol braided stents of3.3 mm internal diameter (Group 2, “3.3 mm stented vein grafts”), foreach of two time points (6 weeks and 12 weeks). The 3.3 mm stented veingrafts and 6.6 mm stented vein grafts were implanted bilaterally in anyone of 8 animals. The stented veins were down-sized to 73% of theunrestricted outer diameter in venous circulation in the 3.3 mm groupwhile the stents were over-sized by 40% of the unrestricted outer veindiameter in the 6.6 mm group.

Protocols

Stents were made from a Nitinol (NiTi)-wire tubular braid material,assumed to be non-compliant in the radial direction and thus, notpermitting any change in diameter due to changing blood pressure. Thestents were manufactured according to the specifications required forthe study. Since the principal stent parameter in this study was thesize (=diameter) of the stent, manufacture was aimed at achieving stentsof different size but the same, or at least sufficiently similar,mechanical properties, especially compliance. The stents of differentdiameter developed for this study exhibit the same pitch (distance)between adjacent Nitinol wires, i.e. the same number of wires per unitlength, and the same pitch angle to obtain similar stent properties. Toachieve consistency in pitch distance and pitch angle but differentstent diameters, the number of carriers (wires) in the braid was variedfor each stent. The 3.34 mm stents have 24 carriers, and the 6.68 mmstents have 48 carriers. The wire thickness of 0.05 mm (0.002″) wasdetermined according to manufacturing requirements (Medtronic AVE,Danvers Mass.). The pitch distance and pitch angle was chosen inaccordance with the 36-carrier stents with 0.025 mm as used in Example 1above. Hence, it is assumed that the compliance of the 3.34 mm stentsand 6.68 mm stents used in this study are in the same range or lowerthan that of the 36-carrier stents used in the study described inExample 1 above.

Surgical protocols, explantation, sample processing, and vein graftassessments were essentially as described in Example 1, above. The 3.3mm stented vein grafts and 6.6 mm stented vein grafts were implantedbilaterally in any one of 8 animals following preoperativerandomization. The mean outer diameter of the superficial femoral veinswere 4.5±0.5 mm and 4.6±0.1 mm in the 3.3 mm stented group and 4.6±0.6mm and 4.8±0.1 mm the 6.6 mm stented group; indicating the degree ofdown-sizing of the vein in the 3.3 mm group and of over-sizing of thestent in the 6.6 mm group. The mean length of the vein grafts was50.8±4.5 mm and 44.0±3.6 mm (3.3 mm stent group), and 51.0±5.4 mm and43.8±4.1 mm (6.6 mm stent group). The lumenal cross-sectional a

It should be noted that the diameter of the stent at implantation wasmeasured at the graft diameter after cross-clamp release (see protocolsin Example 1, above), while stent diameter at explantation was measuredby means of microscopic image analysis of histological slides of crosssectional midgraft sections.

Graft Patency

There was no change in patency between vein graft size or over time.Graft patents was 75% (¾) in all experimental groups, independent ofvein graft size (3.3 mm vs. 6.6 mm) or implant duration (6 weeks (42days) vs. 12 weeks (84 days).

Data from Midgraft Region and Anastomotic Regions

Midgraft region: The lumenal cross-sectional area was 9.0±0.3 mm2 (42days) and 7.4±0.3 mm2 (84 days) in the 3.3 mm group and 22.2±5.0 mm2 (42days) and 23.6±1.5 mm2 (84 days) in the 6.6 mm group. Intimalhyperplasia (IH): The mean IH thickness was 1.6±1.6 μm (42 days) and11.3±5.7 μm (84 days) in the 3.3 mm vein grafts and 112.6±62.6 μm (42days) and 158.5±34.0 μm (84 days) in the 6.6 mm vein grafts.Cross-sectional IH area was 0.02±0.02 mm2 (42 days) and 0.11±0.06 mm2(84 days) in the 3.3 mm vein grafts and 2.0±1.0 mm2 (42 days) and2.9±0.9 mm2 (84 days) in the 6.6 mm vein grafts. The difference of IHarea between the 3.3 mm group and the 6.6 mm group was statistically notsignificant at 42 days (p=0.11) but significant at 84 days (p=0.04).Media: The mean thickness of the media layer was 40.1±10.5 μm (42 days)and 22.8±3.5 μm (84 days) in the 3.3 mm vein grafts and 197.9±65.2 μm(42 days) and 156.8±39.7 μm (84 days) in the 6.6 mm vein grafts. Thecross-sectional media area was 0.5±0.1 mm2 (42 days) and 0.22±0.03 mm2(84 days) in the 3.3 mm group compared to 3.6±0.9 mm2 (42 days) and2.9±0.9 mm2 (84 days) in the 6.6 mm group. The difference incross-sectional media area between the two groups was statisticallysignificant at 42 days (p=0.025) and not significant at 84 days(p=0.063). Adventitial hyperplasia (AH): The mean thickness ofadventitial hyperplasia, defined as elastic tissue development betweenstent and adventitia, was 198.3±49.0 μm (42 days) and 125.1±9.9 μm (84days) in the 3.3 mm vein grafts and 414.0±142.6 μm (42 days) and545.4±100.6 μm (84 days) in the 6.6 mm vein grafts. The cross-sectionalAH area was 2.0±0.5 mm2 (42 days) and 1.1±0.2 mm2 (84 days) in the 3.3mm group compared to 8.0±3.0 mm2 (42 days) and 8.6±1.2 mm2 (84 days) inthe 6.6 mm group. The difference of AH area between of 3.3 mm grafts and6.6 mm grafts was statistically not significant at 42 days (p=0.12) butsignificant at 84 days (p=0.003).

Proximal anastomosis: Mean pannus thickness was 29.2±24.4 μm (42 days)and 47.1±25.8 μm (84 days) in the 3.3 mm vein grafts and 357.2±219.3 μm(42 days) and 105.5±44.7 μm (84 days) in the 6.6 mm vein grafts. Meanmedia thickness was 44.5±22.9 μm (42 days) and 78.8±16.4 μm (84 days) inthe 3.3 mm group compared to 118.4±11.1 μm (42 days) and 134.4±35.4 μm(84 days) in the 6.6 mm group. Mean AH thickness was 157.3±44.5 μm (42days) and 225.5±57.2 μm (84 days) in the 3.3 mm group compared to657.3±132.1 μm (42 days) and 503.9±117.8 μm (84 days) in the 6.6 mmgroup.

Distal anastomosis: Mean pannus thickness was 45.8±33.9 μm (42 days) and120.2±62.3 μm (84 days) in the 3.3 mm vein grafts and 222.9±76.3 μm (42days) and 199.1±30.2 μm (84 days) in the 6.6 mm vein grafts. Mean mediathickness was 75.5±5.7 μm (42 days) and 156.7±78.9 μm (84 days) in the3.3 mm group compared to 191.2±30.0 μm (42 days) and 132.4±32.6 μm (84days) in the 6.6 mm group. Mean AH thickness was 228.4±82.9 μm (42 days)and 355.5±39.3 μm (84 days) in the 3.3 mm group compared to 660.1±96.1μm (42 days) and 505.5±190.8 μm (84 days) in the 6.6 mm group.

Summary

Comparison of vein grafts in the 3.3 mm stent group versus the 6.6 mmstent group showed no difference in graft patency. Macroscopically, theintima was very delicate in 3.3 mm stent group but thick and white in6.6 mm stent group. Endothelial damage was exclusively confined to 6.6mm stent group. Circular micro-folds of the intima were found mostly in3.3 mm stent eSVS group.

Results from this study can be compared with results from the studydescribed in Example 1 above, to assess additional factors. Intimalhyperplasia (IH) was more pronounced in oversized 6.6 mm stent group ofthe present study than in non-stented control group of Example 1.Intimal hyperplasia continued to increase (cross sectional area wise)beyond 6 weeks in the stented vein grafts of the present study, incontrast to non-stented controls vein grafts of Example 1. Downsizing ofvein grafts with 3.3 mm stents was showin in the present study to be asgood as using stents matching the ideal vein sizing, i.e., the 5 mmstents used in Example 1, above.

In the down-sized stented grafts, i.e., the 3.3 mm stent group, nointimal hyperplasia (IH) was measured at 6 weeks, and very limitedintimal hyperplasia was measured at 12 weeks. Very thin and delicatemedia was seen. Atrophy happened rapidly during first 6 weeks with nofurther “thinning” of the media thereafter. No adventitial hyperplasia(elastic tissue development between stent and adventitia) was measured.The inner surfaces of the veins had confluent endothelium.

In the over-sized stented grafts, i.e., the 6.6 mm stent group, intimalhyperplasia (IH) was distinct (⅓) to mild (⅔) at 6 weeks, and mild (¼)to moderate (¼) to distinct (¼) to massive (¼) at 12 weeks. Clearelastic demarcation between intimal hyperplasia layer and media was seenboth at 6 and 12 weeks. Media layer was more pronounced than innon-stented vein grafts (“control” group) of Example 1, above. The innersurves of the veins had semi-confluent to completely denudedendothelium.

Thus, downsizing of vein grafts with external mechanical support bymeans of braided Nitinol stents (here, 3.3 mm stents) considerablylimited the intimal and adventitial hyperplastic response, and preventedendothelial damage, compared to vein grafts with oversized externalstent support (here, 6.6 mm stents). Intimal and adventitialhyperplastic response in downsized vein grafts was of a lower degreethan in vein grafts with size-matching external support and non-stentedvein grafts shown in Example 1, above. Intimal hyperplasia (IH) in veingrafts with oversized external support exceeded the intimal hyperplasiain non-stented vein grafts shown in Example 1, with the development ofIH tissue in grafts with oversized support progressing beyond 42 daysimplantation, while it stagnated after 42 days in non-stented veingrafts. Downsizing of vein grafts with external stents was found to besuperior to oversized external support, and as good as externalmechanical support matching the size of the vein in venous circulation.

EXAMPLE 3A Determination of In Vivo Compliance of External SaphenousVein Stented Grafts Via Ultrasound Measurements in Canine Femoral GraftModel System

A two-factor study varied graft design and implant duration, i.e.,implanting a non-stented vein graft (also called “vein alone”) or astented vein graft with, for different implant durations. Ultrasoundexamination was used to show that a saphenous vein graft supported by aradially-compliant external stent results in an in vivo graft whichcontinues to show radial compliance throughout the healing phase.Briefly, four (4) canines were implanted, each with one non-stented veingraft and one stented vein graft. Ultrasound measurements were made at2, 4, 8, and 12 weeks after implantation to track graft patency and toestimate in vivo compliance values. At 12 weeks after implantation,grafts were explanted and prepared for histological evaluation.

Protocols:

Four (4) adult female canines were implanted with autologous vein graftsas described below. Each animal was implanted with one non-stented veingraft and one stented vein graft. The implanted vein grafts were removedfrom the femoral venous position of each animal and transposed to thecorresponding lateral arterial position. The position of the non-stentedand stented vein grafts were alternated between left and right femoralpositions, from one study animal to the next.

The tubular members, or stents, used in the study were knitted nitinol(NiTi) wire, even knit design, open mesh tubes with 3.3-mm ID. A “#2model” stent was made from a 0.05 mm OD nitinol wire. A “#6 model” stentwas made from a 0.0375 mm OD nitinol wire product.

All current standard operating procedures (SOP) for the study site(Physiological Research Laboratories (PRL), Medtronic, Inc.,Minneapolis, Minn.) were followed, compliance with comply with theAnimal Welfare Act of 1966 (P.L. 89-544), and all amendments, andregistrations and accreditations with agencies and organizationsinvolved in laboratory animal welfare, and in adherence to laboratoryanimal welfare principles stated in The Guide for the Care and Use ofLaboratory Animals U.S. Department of Health and Human Services ,Institute of Laboratory Animal Resources , Commission on Life Sciences,National Research Council, National Academy Press (Revised 1996, ISBN0-309-05377-3; NIH publication no. 86-23).

Vein Graft Procedures

With the animal in supine position, an incision was made in each groinand the superficial femoral vessels between the deep femoral arteryproximally and branching sites distally were carefully dissected outwithout touching them, in an effort to prevent spasm. Once sufficientlyexposed, the diameter of the segment of vessel to be transplanted wasdetermined by approximating the jaws of a vernier caliper, and thesevalues recorded. A lower magnification photograph showing both in situvessels was taken, and a higher magnification photograph of each in situvessel by itself was taken After photography, 250 U/kg of IV heparin wasadministered and allowed to circulate. A 5 cm segment of the veinbetween the proximal and distal branching sites was measured andcircumferentially marked with methylene blue. Vascular clamps wereapplied both proximally and distally, and the marked segment wasexcised. An additional 1 cm of the remaining proximal vein was taken,placed over a 5 mm pipette (to keep vein dilated) and fixed in formalinfor control histology. A 5 cm segment was marked on the artery beforeapplying the clamps, to control for possible distortion of the length ofvessel excised, due to longitudinal contraction of the artery uponexcision. The distance from the deep femoral artery was at least 1 cm toaccommodate the anastomosis-protecting extra length of the stent.

Fixed left and right femoral vein segments collected as described abovewere preserved as “controls” for comparison with the stented grafts.Additionally, a proximal 1 cm portion of each excised femoral arterysegment was removed and placed in fixative, to provide arterial tissuecontrols.

Placement of the Stent

Clamps were applied to the artery proximally and distally, and themarked artery segment was excised. The proximal end-to-end anastomosisof the reversed femoral vein to femoral artery was performed with arunning suture of 7/0 Prolene. When this procedure was completed, a 7 cmlength of the stent was cut. A single stay suture of 7/0 Prolene wasapplied to the adventitia of the vein and then fed through the stent,whereafter the stent was gently slid over the vein graft while using thestay suture as a guide. The distal anastomosis was completed with arunning suture of 7/0 Prolene. Before tying the suture, the graft wasflushed, first from the distal end, then from the proximal end. Theclamps were then removed and flow was re-instated. At this time, anyanastomotic leaks were addressed. Once hemostasis of the tension-freeanastomosis was verified, and flow in the anastomosed vessels wasverified by inspection and palpation, the stent was fixed approximately1 cm proximally and 1 cm distally to the anastomosis with twoadventitial stay sutures on either side.

Immediately after implantation, each device was photographed in situ(“implant macro-photographs”) to document the left- or right-sidednature of the implant and the degree of anastomotic size matching, whilethe implant ID#, the animal number, and the date of implantation werenoted on the photograph worksheet. The diameter of the grafted vesselswas measured at that point and recorded, and the diameter was recordedagain approximately ten minutes after removal of the clamps.

Placement of Non-Stented Vein Grafts

Non-stented vein grafts, i.e., vein grafts without the stent, wereimplanted in each contralaterial position, according to the proceduredescribed above, minus application of the stent over the adventitia ofthe vein. For non-stented vein grafts, each animal served as its own“control” as the grafted vein segment was removed from the femoralvenous position of each animal and transposed to the correspondinglateral arterial position of the same animal.

Closure and Post-Implantation Care

The groin incisions were then closed in two layers with absorbable 2/0Vicryl sutures for subcutaneous closure, and 2/0 Nylon for skin closure.Following implantation, anticoagulants were administered to each animal,and antibiotics were administered as deemed necessary pursuant toongoing veterinary supervision.

Termination, Explantation, and Preparation of Specimens for Histologicaland Scanning Electron Microscopy Analysis

After the final in vivo ultrasound measurement (under anesthesia) wascompleted at 12 weeks, animals were terminated. Animals wereadministered acepromazine, and induce using either a short-actingbarbituate or a short-acting hypnotic, and maintained on isoflurane. Forfemoral venotomy, the animal was placed on its back. The proximal iliacarteries or superficial femoral artery were isolated and perfusion lineswere inserted.

In-Situ Perfusion Fixation (IPF)

For IPF, as applied to biomedical implant retrieval, fixative wasinjected into the vascular system at physiological pressure to preservethe device and associated tissue(s) or organ(s) in the in-vivo ornatural configuration. Briefly, IPF on the lower extremity vasculatureinvolves exposure of the proximal superficial femoral artery (SFA).After administration of 500 I.U. heparin per kg following a lethalpotassium chloride bolus injection and confirmation of cardiac arrest,the SFA was ligated proximally and distally of the implanted vasculargraft. Within the ligations, a perfusion entrance line was tiedproximally and a perfusion exit line was placed distally of theimplanted graft for release of perfusion fluids to a drain. A clearingbuffer solution was delivered by pressure controlled delivery into theSFA, to clear all the blood from the lower extremity. The injectionpressure was monitored via a pressure line connected via athree-way-stopcock at the entrance line. In this manner, IPF atapproximately 100 mmHg was initiated within 5 minutes of cardiac arrest.Shortly after the exit fluid became clear with clearing buffer solution,the clearing buffer was replaced by buffered formaldehyde fixativesolution. The tissues rapidly become very stiff from the cross linkingof the tissues by the fixative. The device(s) and/or area(s) of interestare then carefully removed en bloc while avoiding excessive handling andair-drying of the explant specimen. The explant was immediately rinsedin the same buffer and submersed under the same formaldehyde fixativefor storage and shipping. A standard histological workup of stentedgraft explants.

Graft Patency: Determination of Vein Graft Diameters and Compliance InVivo Using Ultrasound Measurements

The original study plan proposed recording ultrasound measurements ofthe proximal artery, proximal anastomosis, mid-graft, distalanastomosis, and distal artery. Due to concerns that a vascular probecould damage the internal wall of the vessels immediately after thetransplant, different ultrasound measurements were taken.

Pre- and post-implantation compliance, size and patency (vessel diameterand occlusion/dilation) of each non-stented vein graft and each stentedgraft were measured using the vascular probe on a GE ultrasound machineunder anesthesia. Ultrasound measurements of A_(systole), D_(systole),A_(diasystole), and D_(diasystole) were made for each non-stented veingraft and each stented vein graft, at pre-implantation, and at 2, 4, 8,and 12 weeks post-implantation, at the following locations: proximalartery (PArt); proximal anastomosis (PAna), midgraft (MG), distalanastomosis (DAna), and Distal Artery (DArt). Ultrasound measurementswere an average of at least two determinations using ultrasound vascularplanimetry software.

Diameter (D) at each location was calculated from the A (lumencross-section area) value measured at that location (A_(location)) usingthe formula A_(location)=πD²/4. Vein diameters were generally higher inthe non-stented vein grafts than the stented vein grafts.

Compliance at each location was calculated as follows: %Compliance={[D_(systole)−D_(diastole)/D_(diasystole)]}/[(P_(systole)−P_(diasystole))/100]}*100,where D is diameter, and P is pressure. Compliance data measured in vivoindicated that compliances for both the non-stented vein grafts andstented vein grafts, in the mid-graft region, appeared to fall in the5-15 %/100 mm Hg range. Based on literature values suggesting naturalvessel compliance to be in the range of 4-10%/100 mm Hg, with lowervalues for veins than for arteries, the in vivo compliance valuesmeasured for vein grafts were considered to be within the range ofcompliance values of natural vessels.

These in vivo compliance measurements were compared with known in vitrocompliance measurements of 8-21 of the #2 model stents and 21-29 of the#6 model stents used in this study. It was expected that implantinghighly radially-compliant stents may invoke more natural healing.Specifically, the known in vitro compliance values, combined with a lowlevel of scarring after grafting, was expected to potentially create agraft with an in vivo compliance levels consistent with a native artery.A more “physiological” level of graft compliance was thought to be acrucial factor in creating an arterial-like reorganization of the venoustissue, which would result in high graft patency.

Evaluation of Explanted Vein Grafts

As described above, animals were euthanized 12 weeks post-implantation(77 days), following the final ultrasound measurement. The grafts weresubject to gross examination, explantation, and evaluation by variousmethods including histological evaluation, e.g., light microscopy ofstained fixed sections or scanning electron microscopy (SEM) of fixedsections, or evaluation of faxitron (radiograph) images of grafts.Photographs were taken of intact grafts before excising, explantedgrafts, and fixed vessels and vessel sections in certain animals.

Gross examination of explanted non-stented and stented vein graftsshowed similar smooth, glistening lumens. In the explanted stented veingrafts, the stent structure was clearly visible through the thin veinwall. Some regions of stent wire disruption could be seen in the lumenalview of the stented vein graft explants.

Fixation and Histological Evaluation

Histological examination showed that the healing response was verysimilar between the two types of stents, i.e., the #2 model and the #6model. Little, or no, inflammatory response to the metal stents wasobserved. The diameters/cross sectional areas were measured, and bygraphical analysis of the measurement, it was determined thatnon-stented vein grafts (controls) had slightly larger diameter/crosssectional area values than the stented vein grafts. Thediameters/cross-sectional areas appears similar in each type of graft,regardless of the stent employed, i.e., the #2 model or the #6 model.Endothelization, measured as % endothelial cell coverage, was similar(at or near 100%) in stented and non-stented grafts.

Resin-embedded explant sections were actin-stained for vessel wallmeasurements and evaluation of structures, e.g., wall thickness andextent of elastin cushioning. Intimal hyperplasia and medial hyperplasia(mean medial thickness) were measured as an indicator of wallthickening, which was considered to correlate with proliferation and/orinjury. Low levels of intimal or medial hyperplasiea were observed withthe metal stents.

Scanning electron micrographs (SEM) of all grafts, stented andnon-stented, showed relatively intact endothelium on all explanted veingrafts.

Faxitron (Radiographic) Evaluation

Faxitron (radiographic) images revealed the extent of non-stented veingraft size and dilation, relative to the femoral artery to which it wasattached. The stented vein grafts showed a certain degree of dilation incertain locations. Careful inspection of the images showed that thedilation was mainly located in the portion of the stent covering thevein graft. The portion of stent overlapping the proximal and distalartery did not appear to show dilation. Regions of the dilating portionof the stent showed a small amount of wire breakage and/or disruption inthe normal knitted pattern. The latter observations appear consistentacross each stent design, i.e., the #2 model and the #6 model.

Evaluation of the particular stent designs used in this study, i.e., the#2 model and the #6 model, indicated that these stent designs appear toshow some level of in vivo radial ‘creep’ which is inconsistent withother similar implants. The reason for the observed radial creep was notdetermined, but it was speculated that loop design andsupra-physiological compliances of these designs may be a factor. Otherimplants utilizing an uneven loop design, e.g., the “K1 design”extensively tested in other studies, did not show radial creep asobserved here. The #2 model and the #6 model stent designs had unevenloop design, with 8 loops per circumference, with loops alternatingbetween an large and small size, creating an alternating geometry thatmay make the stent slightly more rigid, in addition to adding stabilityto the structure. However, further investigation is necessary to ruleout other potential sources of the problem observed here, e.g., poorquality wire lots and insufficient annealing.

EXAMPLE 3B Surgical Methodology and Healing Response Characterization ofthe Stent Device in the Coronary Vasculature in a Canine Model

The study had a three-fold purpose to develop surgical methodology,characterize the healing response, and determine if one or both stentedgrafts perform better than the non-stented control vein graft. Morespecifically, one goal was to develop and refine the best methodology toapply a stented graft in the aorto-RCA coronary or aorto-LAD positionsuch that it optimally covers and is attached to the proximal and distalanastomotic regions. A second goal was to contrast the healing responseof the stented grafts versus the non-stented control vein graft in termsof healing and patency.

This study successfully employed a surgical methodology in a caninemodel to apply the stent during a coronary artery bypass grafting(“CABG”) procedure. It was determined that the femoral vein was moresuitable in this canine model than the saphenous vein, due to vesselsize and fit within the stent. Briefly, five female dogs underwent afemoral vein graft harvest followed by a beating heart aorto-coronarybypass with four (4) animals receiving a stented vein graft, and two (2)receiving the autologous femoral vein without a stent. Vein grafts weremonitored in vivo following implantation. Twelve weeks after vein graftimplantation, the vein grafts were removed.

The stents used in the study were K1 model knitted NiTi (Nitinol) wiremesh compliant stents, uneven design, 7 cm or greater length, 3.3 mm ID(Danvers Model K1, Medtronic Vascular, Danvers Mass.). The stents weresupplied mounted on 9 Fr removable plastic tubes, where the tubes wereprovided for facilitating vein graft insertion through the stent lumen.The “control” articles were autologous femoral vein without a stent, andthe control treatment is the vein graft without a stent, i.e., the“non-stented vein graft.”

Surgical Procedures

The femoral vein was harvested from the right leg of each animal. Aftercompletion of the harvest the leg was closed. The distal end of theharvested vein was marked.

The CABG procedure was performed as an off-pump procedure (BeatingHeart) utilizing Medtronic Starfish™ and Octopus ™ products to stabilizethe heart during the operation. The proximal anastomosis of the veingraft to the ascending aorta was performed first, using a partialside-biting clamp to the aorta. The artery was momentarily clamped whilethe arteriotomy was performed and inserted into the coronary artery toensure distal perfusion while the anastomosis was being performed. Oncecompleted, the left anterior descending (LAD) or right coronary artery(RCA) was ligated just proximal to the anastomosis. After completing theproximal anastomosis, a suture was tied to the distal vein tissue andthe vein was carefully pulled through the stent lumen. The distalanastomosis was completed, with optional stay sutures added to thisanastomotic region to prevent stent dislodgement.

The vein graft control was implanted applying the procedure describedabove without the stent applied to the grafted vein.

Three animals received a stent over a femoral vein graft (approximately7 cm length), one animal served as an autologous non-stented femoralcontrol, and one animal received an autologous saphenous vein graft. Toexamine the ease and need of stay sutures to hold the stent in placeover the vein graft, some stents received 2-4 stay-sutures at theanastomosis.

Photographs were taken of implanted stented (test) grafts and control(non-stented) grafts showing: (1) image of entire graft on heart andrevealing both anastomoses; (2) a close-up image of the anastomosis tothe aortic arch; and (3) a close up image of the anastomosis to thecoronary artery. In addition, each procedure was videotaped.

All dogs recovered from surgery. Two of the five dogs fibrillated duringthe surgery, one immediately upon reperfusion of the heart, and one justprior to closing the thoracotomy. Both dogs were successfullyresuscitated using defibrillation and drug administration. One dogreceived a saphenous vein graft. Here, some difficulty was encountereddue to the small size of this vessel. As a result, all subsequent dogsreceived a femoral vein graft. All animals had minimal swelling/edema inthe limbs where the femoral or saphenous vein was harvested. Three dogsremained on procainamide for 3-5 days post-operatively due toarrhythmias noted on their daily ECG's. Release of cross clamps beforepositioning of the stent prevented optimum positioning of the stent. Theneed for placement of the stay sutures was unclear at implant, as theblood-filled vein graft appeared to hold the stent in place. The staysutures were technically difficult to apply.

The implants (grafts) were examined by ultrasound monitors that tookplace at 2, 4, 8, and 12 weeks after graft implantation.

At 12 weeks after implantation (grafting) and prior to sacrifice, thegrafts were examined by in vivo angiography and intravascular ultrasound(IVUS). Following the ultrasound monitor, the heart was perfused via acardiac perfusion fixation chamber and perfusion-fixed in situ and thegrafts removed en bloc. Fixed explants underwent Faxitron imaging(radiography), gross photography, SEM of lumenal surfaces, andhistopathology using routine paraffin and plastic resin preparation.

EXAMPLE 4 External Saphenous Vein Stents: Effect of Radial Complaince onVein Healing Response

Sixteen (16) Chacma baboons were implanted with a total of thirty-two(32) vein grafts in a three-factor study to evaluate the effects andinteractions of stent design (braided vs. knitted), radial compliance(low (non-crimped) vs. high (crimped)), and implant duration (6 weeksvs. twelve weeks).

In this study design, radial compliance of a stent of a given design wasadjusted by crimping or not crimping the stent, where crimping increasedthe radial compliance of both stent designs, in accordance withdisclosures elsewhere in the application (See, e.g., Table III). Thatis, a braided stent material (B1 design, Medtronic AVE, Danvers Mass.)having a low radial compliance value was used for the braided stents,where high compliance braided stents resulted from crimping the braidedstent material, and low compliance braided stents resulted when thebraided stent material was left alone (non-crimped). The samerelationship applied for the knitted stents (K1 design, Medtronic AVE,Danvers Mass.). Thus, the stents used for this study included (1)non-crimped braided stents with a 3.3 mm ID having low radial complianceof about 1%/100 mmHg, or “low compliance non-crimped braided stents”;(2) crimped braided stents with a 3.3 mm ID, having high radialcompliance of about 6%/100 mmHg, or “high compliance crimped braidedstents”; (3) non-crimped knitted stents with a 3.3 mm ID having lowradial compliance of about 3-4%/100 mmHg, or “low compliance non-crimpedknitted stents”; and (4) crimped knitted stents with a 3.3 mm ID, havinghigh radial compliance of about 9%/100 mmHg, or “high compliance crimpedknitted stents.” In this study, To ensure the connection betweencrimping

Each animal was implanted was implanted with one high compliance stentand one low compliance of the same design (knitted or braided), andplacement of the high and low compliance stents was randomized betweenright and left femoral artery position. Animals were grouped in “blocks”of four (4) animal per block, such that each block was sufficient toachieve all combinations of stent design (knitted or braided) and radialcompliance (high or low), for one implant duration (6 weeks or 12weeks). Two (2) blocks having an implant duration of 6 weeks and two (2)blocks having an implant duration of 12 weeks achieved fullimplementation of the experimental design. At 6 weeks or 12 weeks afterimplantation, grafts were evaluated for patency and explanted for visualand histomorphological analysis and ultrastructure analysis by scanningelectron microscopy (SEM).

Baboons were chosen for these studies since the size and anatomy of thecardiovascular system were considered clinically relevant for thepurpose of externally stented vein graft testing. The diameter of thefemoral vessels approximates the nominal 3-4 mm ID of the test veinstents being developed for human use. This model is also generallyacknowledged to show a healing response similar to humans, asdemonstrated in previous stented vein grafts studies described hereinand elsewhere. Because stent performance is influenced by the degree towhich the stent fits over the saphenous vein it surrounds, the weightrange of the Chacma baboons was restricted to 18+/−5 kg.

Treatment of study animals was in compliance with the Animal Welfare Actof South Africa and subsequent amendments, and pursuant to accreditationby the Association for Assessment and Accreditation of Laboratory AnimalCare, International (AAALAC), and was further in adherence to animalwelfare principles stated in The Guide for the Care and Use ofLaboratory Animals—National Academy Press (Revised 1996, NIH publicationno. 86-23).

The test materials used for stents were: eight (8) low compliancenon-crimped braided stents; eight (8) high compliance crimped braidedstents; eight (8) low compliance non-crimped knitted stents, and eight(8) high compliance crimped knitted stents As noted above, all stentshave a 3.3 mm ID.

Protocols

All stents were cut to the length of 70 mm using surgical scissors,sterilized, and labeled. Prior to surgery, each animal was anesthetized,intubated with a endotracheal tube, and the operative fields wereshaved.

With the animal in supine position, an incision was made in each groinand the superficial femoral vessels between the deep femoral arteryproximally and branching sites distally were carefully dissected outwithout touching them in an effort to prevent spasm. Once sufficientlyexposed, the diameter of the segment of vessel to be transplanted wasmeasured by approximation using a vernier caliper. A lower magnificationphotograph showing both in situ vessels and a higher magnificationphotograph of each in situ vessel was taken. After 250 U/kg of IVheparin was administered and allowed to circulate, a 5 cm segment of thevein between the proximal and distal branching sites was measured andcircumferentially marked with methylene blue. Vascular clamps wereapplied both proximally and distally and the marked segment will beexcised. Another 1 cm of the remaining proximal vein was taken, placedover a 5 mm pipette and fixed in formalin for histological analysis of acontrol sample. In a similar fashion, a 5 cm segment was marked on theartery before applying the clamps, to accommodate possible distortion ofexcised length due to longitudinal contraction of the artery uponexcision, as the distance from the deep femoral artery must be at least1 cm to accommodate the extra length of the stent that protects theanastomosis.

After clamps were applied to the artery proximally and distally, themarked artery segment was excised and the proximal end-to-endanastomosis of the reversed femoral vein to femoral artery was performedwith a running suture of 7/0 Prolene. The stent was cut to 7 cm (ifnecessary) and a single stay suture of 7/0 Prolene was applied to theadventitia of the vein and then fed through the stent, after which thestent was gently slid over the vein graft using the stay suture as aguide. The distal anastomosis was then completed with a running sutureof 7/0 Prolene. Before tying the suture, the graft was flushed, firstfrom the distal end, then from the proximal end. The clamps were removedand flow re-instated. At this time, any anastomotic leaks wereaddressed. Once hemostasis of the tension-free anastomoses was verified,and flow in the anastomosed vessels was verified by inspection andpalpation, the stents were fixed approximately 1 cm proximally anddistally to the anastomoses with two adventitial stay sutures on eitherside. The implant macrophotographs were taken at this point. Thediameter of the grafted vessels was measured again approximately tenminutes after removal of the clamps. The groin incisions were closed intwo layers with absorbable 2/0 Vicryl sutures for subcutaneous and 2/0Nylon for skin closure.

Immediately after implantation, each device was photographed in-situwith a high quality digital camera, to document the left- or right-sidednature of the implant, the degree of anastomotic size matching, theimplant ID#, the animal number, and the date of implantation.

First, results from the present study were analyzed by analysis ofvariance (ANOVA) with respect to each factor, and with respect to factorinteractions in the full factorial study. Next, ANOVA analysis wascarried out on results from the present study, combined with the resultsfrom the non-stented “control” group from the study described in Example1 above, to gain additional perspective on the results and theirimplications. This appreach was considered reasonable since the healingresponse(s) of each type of stented vein graft ultimately needs to becompared not just to other stented vein grafts within the present study,but also to a “normal” treatment group that received non stented veingrafts. In addition, the animal model, surgical methods, implantsposition, etc. used in both studies are identical. The main caveatsassociated with this comparison are (1) the level of one variable isdifferent i.e., time point 2 is three months in study in Example 1, andsix months in this study, and (2) the control group of Example 1 was notrandomized into this study and was in fact done many months earlier inthe program.

Assessment of Graft Patency

At the end of the study, there were 11/16 (69%) knitted vein graftspatent and 15/16 (94%) braided vein grafts patent. It was not clearwhether the crimping feature was a factor in graft patency, but itshould be noted that the single occluded braided vein graft was crimped,and 3 out of the 5 occluded knitted vein grafts were also crimped. Thepatency of the control non-stented grafts from the earlier studydescribed in Example 1 was 6/8 (75%). The patency of these short stentedvein grafts was expected to be high in the model used in the presentstudy, and this result was observed. Given the small numbers of graftsinvolved, however, an exact logistic test on the patency data revealedthat the differences were not statistically significant (P<0.0201).Therefore, in this study, patency was not considered a primary indicatorof graft performance. The histomorphological assessments of the graftexplants were therefore viewed as the best overall assessment ofperformance. Specifically, differences in pathological changes in thestented vein graft wall compared to the control non-stented vein graft,received the greatest attention.

As noted above, multiple measurements were compiled usinghistomorphological analyses. Given that many of the measurements areredundant with others, and some measurements are a simple mathematicalmanipulation of another measurement e.g., an area vs. a thickness, thisresults section reviews only three main measurement categories by ANOVAanalysis: Calculated Mean Lumenal Diameter ‘Mic’; Calculated Mean MedialThickness; and Calculated Mean Intimal Hyperplasia. These measurementswere chosen since they were considered to give a strong perspective ofthe patency/blood flow path of the implanted grafts, and a simple,accurate assessment of the degree of pathological thickening that may beoccurring in the two upper-most lumenal layers of the vein graft. Forstatistical analysis of the measurement Mean Intimal Hyperplasia, thedata received a log transformation to minimize variation and normalizethe data.

ANOVA analysis of the present study was performed for the main studyvariables of stent design (braid vs. knit), radial compliance (low(non-crimped) vs. high (crimped)), and implant duration (42 days vs. 180days). The ANOVA results are shown below in Table VIII. TABLE VIIISummary of ANOVA Results Main Study Variables Significant MeasurementeSVS Design (D) Crimping (C) Time (T) Interactions* Calculated Mean NSNS P < 0.0252 DT Lumenal Diameter ‘Mic’ Calculated Mean NS P < 0.0477 NSDT, DCT Medial Thickness Calculated Mean P < 0.0033 NS P < 0.0305 DTIntimal Hyperplasia*P < 0.05 for any listed interactionsNS = not significant

Calculated Mean Lumenal Diameter ‘Mic’ Results

These results showed that in general over time i.e., implant duration,Calculated Mean Lumenal Diameter ‘Mic’ significantly increased. Theoverall mean lumenal diameter increased from 3.4 mm to 3.8 mm over theimplant period. This was likely due to a gradual stretch/relaxation inthe stent mesh structures. The significant Design-Time (DT) interactionobserved here was that the vein grafts with knit design stents showed arelatively stable lumenal diameter over the implant period whereas thevein grafts with braid design stents showed a tendency for the lumenaldiameter to increase over the implant period. ‘Crimping’ i.e., increasedradial compliance introduced by crimping the stents, did notsignificantly influence vein graft lumenal area, as was dramaticallyillustrated in certain explant images wherein the crimped stentsretained their ‘star-like’ morphology in vivo.

Calculated Mean Medial Thickness

As seen in Table 1, the results in this measurement category indicatedthat the crimped (high compliance) stents are generally associated withsignificantly more medial thickness than the non-crimped (lowcompliance) stents. A significant Design-Time (DT) interaction was alsoobserved for this measurement, which reflects the observation that thebraid stent design showed less medial thickness over the study period,while the knit stent design showed more medial thickness. See FIG. 4.The DCT interaction reflects that the data shows, in the absence ofcrimping, that medial thickness associated with both eSVS designs is thesame over time (FIGS. 5 and 6 from JMP and DesignEase, respectively).However, with crimping, medial thickness over time becomes morepronounced on the Knit eSVS and less pronounced in the Braid eSVS (FIGS.4 and 7 from JMP and DesignEase, respectively).

Calculated Mean Intimal Hyperplasia

As seen in Table VIII, the results in this measurement categoryindicated both the stent Design and Time influenced the extent ofintimal hyperplasia, and that the trends were different in associationwith each design. Here it is seen that the amount of intimal hyperplasiaassociated with each stent design is similar at the early timepoint butthat the amount is significantly higher in association with the knitstent at the later time point.

Examination of Responses in Comparison to Control Non-Stented Group fromthe Study in Example 1

The material above describes the healing response observations betweeneach type of stented vein graft. This section now describes theseresults in comparison to the normal treatment group i.e., non stentedvein grafts. For this, the data from the control non-stented group fromstent Study 1 has been merged into the data for this study. Once again,it must be noted that the animal model, surgical methods, implantsposition etc. used in both studies are identical. However, caveats withthis comparison are (1) the level of one variable is different i.e., thelate time point 2 in stent Study 1 is 3 months rather than the 6 monthas in stent Study 3-4, and (2) the control group was obviously notrandomized into Study 3-4 and was in fact done many months earlier inthe program in a separate block of study animals.

It should also be noted that in this secondary analysis the ‘crimping’variable received special consideration and was ultimately excluded. Toexplain: the crimping feature was added to the overall investigation tosee if any significant meaningful improved healing observation(s) couldbe associated with this feature. As Table 1 indicates, crimping had noimpact on calculated mean luminal diameter or extent of intimalhyperplasia, but was seen to significantly (negatively) influence medialthickness. Therefore, crimping is viewed as a feature with no apparentbenefit. It was possibly also associated with stent wire breakage, asbroken stent wires were suspected in 8 knitted stents, 6 of which werecrimped. With crimped stent designs excluded, the overall study takesthe form of a simple 3×2 full factorial design looking at stent/graftdesign (knit(K) vs. braid(B) vs. control(C)) and time (early (T1) vs.late (T2)). Table IX below summarizes the significant ANOVA findings.TABLE IX Summary of ANOVA Results with Comparison to Control Grafts MainStudy Variables DT Measurement stent Design (D) Time (T) InteractionCalculated Mean P < 0.0001 P < 0 0014 P < 0.0100 Lumenal Diameter ‘Mic’Calculated Mean P < 0.0001 P < 0.0004 P < 0.0002 Medial ThicknessCalculated Mean P < 0.0001 NS P < 0.0116 Intimal Hyperplasia

Comparison of Calculated Mean Lumenal Diameter ‘Mic’ Results IncludingControl Non-Stented Grafts

With ANOVA analysis (Table IX) showing each main study variable and theinteraction to be significant, and other results showing that in generallumen diameter associated with the control and braid groupssignificantly increased over time and that the lumen diameter of theknit group stayed relatively constant over this period.

Comparison of Calculated Mean Medial Thickness Results Including ControlNon-Stented Grafts

ANOVA analysis (Table IX) showed each main study variable and theinteraction to be significant, and other results showed that in general,medial thickness associated with the control was significantly higherthan that observed in the braid and knit groups, particularly at theearly time point. The extent of medial thickness associated with thecontrol group also significantly decreased over time compared to thestented groups. Medial thickness in the later groups remained relativelyconstant over the implant period.

Comparison of Calculated Mean Intimal Hyperplasia Results IncludingControl Non-Stented Grafts

ANOVA analysis (Table 2) showed that the stent design variable and theinteraction were significant, and other results showed that intimalhyperplasia was significantly pronounced in the control group comparedto the stented groups at both time points. The results also showed thatthe level of intimal hyperplasia was low and similar in the stentedgroups and that the level of intimal hyperplasia significantly decreasedover time in the control group.

Summary

The results of this stent study, without comparison to the earliernon-stented control group (Example 1), show a number of significanttrends. The lumenal diameter of the stented grafts tended to increaseover time but this was mainly attributable to the increase in diameterassociated with the braid stent group over time. This change in diameteris a concern since, if dilation would continue, the purpose of the stentto maintain the purpose of the stent to prevent stretch injury to thevein and maintain a specified isodiametric blood flow path would bedefeated. The use of longitudinal crimping to impart additional controlto the blood flow path and the stent radial compliance did not lead toimpressive changes in healing responses. Crimping of the stentsgenerally lead to more medial tissue thickness, particularly in the knitstent group. The latter may have been in part due to broken stent wires,wherein wire damage itself may have started as a result of the crimpingprocess (6 out of 8 suspected observations of broken wires occurred incrimped knitted stents). The crimped braided structure appeared to bemore immune to such breakage. When the medial thickness data of thecrimped knitted stents was removed from the database, braided andknitted stents show similar levels of medial thicknesses (around 50 μ).Intimal hyperplasia thickness was low in both the braided and knittedstents but increased to a significant difference in the knitted stentimplants by the late time point. The significance of this observationshould be considered alongside the remarkably higher levels of intimalhyperplasia observed in control non-stented vein grafts.

The results of the present stent study, with comparison to the earliernon-stented control group (Example 1), also show a number of significanttrends. As expected, the lumen diameter of the control non-stented groupincreased tremendously over time (from around 4.5 to 7.7 mm) compared tothe braid stents (3.1 to 4.2 mm) and knit stents (3.6 to 3.5 mm). Theapparent stability of the knitted stent graft lumen diameter may beimportant. The mean medial thickness in the control non-stented graftswas significantly higher than all other groups at the early time point,but fell to slightly above all groups at the later time point. Thereason for this is unclear, as intuitively one might expect it to remainhigh. Additional testing at longer implant duration may help in theunderstanding of this process (also, recall the T2 control group is 12weeks rather than 24 weeks for the stented grafts). Intimal hyperplasiawas remarkably higher in the non-stented control group compared tostented grafts at both time points, but like medial thickness, it wasslightly lower at the later time point. Patency was reasonably high inall graft groups in this study, but the difference in patency betweenthe groups was found to not be statistically significant.

EXAMPLE 5 External Saphenous Vein Stents: Impact of Stenting by Position(CABG vs. Femoral) and Vein Type (Lesser Saphenous vs. Femoral)

This study was carried out to evaluate the impact of stenting by graftposition (aorto-coronary (CABG) vs. femoral) and grafted vein type(lesser saphenous vein vs. femoral vein). To study the impact of graftposition, only lesser saphenous veins were used. To study the impact ofgrafted vein type, only femoral implants were used. The study designincluded two nested factorial studies: Study #1 (“S#1”) had a 2×2×2factorial design to evaluate stenting (stented vein vs. non-stentedvein), autologous vein type (femoral vs. lesser saphenous), and fibringlue (with and without glue); and study #2 (“S#2”) had a 2×2 factorialdesign to evaluate stenting (stented vein vs. non-stented vein) andimplant position (femoral vs. aorto-coronary). The study confirmed thatexternally stented vein grafts in the coronary position showed reducedintimal hyperplasia, thus improving the patency of coronary arterybypass grafts.

Study Design

In this study, regardless of the factor(s) being evaluated, each animalreceived (a) a lesser saphenous vein graft in the aorto-coronary (CABG)position; and (b) a lesser saphenous vein and a femoral vein graft inthe bilateral femoral positions. Each animal received either stentedgrafts exclusively or non-stented grafts exclusively, for an implantduration of 180 days.

In the aorto-coronary graft position, grafts with a stent alwaysreceived fibrin glue. Non-stented grafts in the aorto-coronary positionwere subdivided in two groups of n=8, where one group received fibringlue and the other group did not receive fibrin glue. In the femoralgraft position, both the stented animal group and the non-stented animalgroup were subdivided, such that half of each group (n=8) receivedfibrin glue and the other half of the animals did not receive fibringlue. Therefore, in S# 1, replication (n=) for these nested studies wasn=8. In S#2, replication (n=) for these nested studies was between n=8and n=16. The study design included the option that, should statisticalanalysis of the results from S#1 show no significant impact of fibringlue, the replication in S#2 replication would become n=16 by factoringin all the non-fibrin-treated femoral grafts.

The study was designed to be carried out using a sample size based onthe expected number of animals and procedures required to produceresults suitable for application of statistical methods for determiningsignificance. Briefly, a suggested sample size of 16 animals per groupwas based on the assumption that 35% of non-stented control grafts and75% of stented grafts remain patent with a 75% probability of discerninga statistically significant difference at a p value of 0.05.

Chacma Baboon Model

Chacma baboons were chosen because the size and anatomy of thecardiovascular system of the Chacma baboon was expected to be clinicallyrelevant for externally stented vein graft testing. The diameter of thesaphenous and femoral vessels approximates the nominal 3-4 mm ID of thetest vein stents developed for this and other studies. Further, theChacma baboon model is generally acknowledged to show a healing responsesimilar to humans, as demonstrated in other preclinical studies. Malebaboons having weights between 29.3±6 3.2 kg were implanted with graftsas described below. The weight range of the Chacma baboons used in theseprocedures was in accordance with the understanding that the performanceof each stent would be influenced by the degree to which the stent fitover the autologous vein it surrounded.

Treatment of study animals was in compliance with the Animal Welfare Actof South Africa and subsequent amendments, and pursuant to accreditationby the Association for Assessment and Accreditation of Laboratory AnimalCare, International (AAALAC), and was further in adherence to animalwelfare principles stated in The Guide for the Care and Use ofLaboratory Animals—National Academy Press (Revised 1996, NIH publicationno. 86-23).

Preparation of Stents for Grafting

At the beginning of the study, forty eight (48) knitted compliant K1model stents having an ID=3.4 mm, length=200 mm, and radial complianceof about 3%/100 mmHg, were provided. The K1 model was knitted, with 8loops per circumference, from a wire having a thickness of 0.002 inches(=0.05 mm), and had uneven loops. In accordance with the study design,32 stents were to be grafted into a femoral position, and 16 stents wereto be grafted in an aorto-coronary (CABG) position, where each positionrequired a different stent length. The stents required for each animalreceiving one or more stented grafts was as follows: one piece of 15 cmlength for the CABG stented grafts; and two pieces of 7 cm length forthe bilateral femoral stented grafts.

The stents were cleaned after heat setting in the following two-stepprocess before the stents were ready for implantation: 1) sonication inde-ionized water (50 ml per 60 cm length of stent, for 5 minutes at roomtemperature); and 2) sonication in isopropanol (50 ml per 60 cm lengthof stent, for 5 minutes at room temperature). After cleaning, the stentswere handled with powder-free latex gloves only.

Prior to implantation, the stents were cut to the lengths specified bythe experimental design. Each stent underwent visual inspection anddocumentation by means of macroscopic photography, capturing images at amagnification of 1.25× along the length of each device in increments of15 mm from end to end. Thereafter, each stent was individually placed onan delivery (assembly) tube. Assembly tubes were polymeric or stainlesssteel hypotubing.

Surgical Methodology for Vein Grafting and Stenting

Animals were prepared according to standard procedures as necessary forfemoral implantation and for aorto-coronary implantation. The lessersaphenous vein was harvested from one leg of each animal and preparedfor graft construction. Briefly, after an incision over the path of thesaphenous vein at the back of the calf muscle, the vein was dissectedfree from surrounding tissue. The vein was then ligated and cannulatedproximally. After injection of blood containing heparin and papavarine,the side branches were ligated and divided. The distal end of the veinwas then ligated and the vein was be tested for any leakage. In the casethat the harvested length of saphenous vein was not sufficient for theconstruction of the aorto-coronary graft and one femoral graft, thelesser saphenous vein from the other leg of the animal was harvested inthe same fashion.

Each animal underwent CABG surgery under full cardiopulmonary bypass.Briefly, the sternum of the animal was opened and cardiopulmonary bypasswas established. In parallel, each vein graft was be prepared accordingto the type of graft called out in the experimental design. In the caseof a stented graft, the saphenous vein and stent were assembled byplacing the stent on an delivery (or assembly) tube, feeding the veinthrough the tube, and then carefully removing the tube while holding thevein and stent in place.

For grafts receiving fibrin glue, the glue was applied to the graft atthis stage by spraying, while the vein was injected gently withpapaverine in blood, in order to gently squeeze or press the veinagainst the inside of the stent. Pursuant to experimental design, allstented CABG grafts and half of the non-stented control CABG graftsreceived fibrin glue.

For the implantation of the vein graft, the distal anastomosis to theleft anterior descending artery (LAD) was constructed first in anend-to-side fashion. After the distal anastomosis was completed, theproximal anastomosis of the vein graft to the aortic ostium wasperformed. For stented grafts, both anastomoses were fashioned eitherwith, or without, incorporating the stent into the anastomosis atdiscretion of surgeon. For incorporation of the stent into theanastomosis, stent and vein were cut flush, and each stent loop wasincluded in the suture stitches. For anastomosis without stentincorporation, the stent was cut 2 mm shorter than the vein, and thestent was not included in the anastomotic stitches. After reinstatingphysiological circulation, the sternum of the animal was closed.

After the CABG surgery was completed, the femoral grafts were implantedbilaterally. Incisions were made to open the groins, and the superficialfemoral vessels were dissected between the deep femoral arteryproximally, and branching sites distally. For the femoral positionreceiving a femoral vein graft, a 5 cm long piece of the femoral veinwas harvested. For the femoral position receiving the lesser saphenousvein as a graft, excision of the femoral vein was not performed.

A 5 cm long segment of the femoral artery was excised and the proximalend-to-end anastomosis of the reversed vein was then performed. Forstented grafts, the stent was placed over the vein at this stage,utilizing a delivery (assembly) tube in a similar fashion as describedabove for stented CABG graft preparation. After this step, the distalanastomosis was performed in an end-to-end fashion. For stented grafts,the stent was positioned such that it covered each anastomosis 1 cmproximally and 1 cm distally, and was secured in this position with twoadventitial stay sutures on each anastomosis.

For replicates where the femoral grafts received fibrin glue, the gluewas applied to the graft by spraying, after both anastomoses of thegraft have been completed. Thereafter, the groin incisions were closed.

Implant Photography

For complete documentation, a set of photographs was taken bothpre-implantation and immediately after implantation (pre-closure) forvein grafts in the femoral position and for vein grafts in theaorto-coronary position. Pre-closure, each vein graft was photographedin-situ to document the general nature of the implant and the degree ofanastomotic size matching.

Termination

At the times indicated, the implanted devices were removed andevaluated. Terminations were scheduled to fall within 3 days of theassigned implant duration. Animals were prepared for femoral vein graftexpalantion, and/or aorto-coronary vein graft explantation according tostandard procedures.

In situ Perfusion Fixation was used prior to explantation according toprocedure described in Example, 1, above, adapted as necessary forfemoral vein grafts and aorto-coronary vein grafts. A complete set ofhistology slides was also prepared.

Graft Excision

Each femoral vein graft (stented and non-stented control) was removed inan en-bloc excision, with attached 1-3 cm native proximal and distalartery tissue according to standard procedures. For each heart with aCABG graft, the heart was excised entirely.

Angiographic Patency Assessment of CABG Grafts

Angiographic patency assessment of all explanted hearts with theaorto-coronary grafts was carried out as described below. Immediatelyafter the explantation of the heart, the ascending aorta was clampedproximally and distally to the proximal anastomoses of theaorto-coronary graft. The proximal vascular clamp was applied just abovethe native coronary ostia to ensure that contrast media would not escapethrough the native coronary vessels or through the aortic valve. Thedistal clamp was applied just beneath the origin of the head and neckvessels on the aortic arch to ensure that the contrast media only flowedalong the aorto-coronary graft (if the graft was patent). Thereafter,the position of the graft was marked using small liga clips on softtissue next to the graft, serving to indicate the position of the graftat the start of the angiography before the injection of any contrastsolution.

The explanted heart prepared in the described fashion with the attachedaorto-coronary graft was subjected to tissue fixation in 4% formaldehydesolution for not less than 2 hours prior to the angiographic assessment.

An interventional angiography system was used for the CABG patencyassessment. The explanted heart was positioned appropriately to ensureunobstructed imaging of grafts. Contrast media was injected via anaortic root cannula inserted at the time of explantation for perfusionfixation of the heart. Non-ionic Iopromide contrast medium (e.g.Ultravist® 300) diluted 50% using 0.9% NaCl solution, was used ascontrast solution. Approximately 20 ml of contrast solution was usedwith each imaging attempt. The views facilitated were mainly apostero-anterior view as well as a right anterior oblique view. Whilethe contrast solution was being injected, the image was captured as adynamic clip, from which appropriate still pictures were selected at alater stage. Angiographic still images were also captured with theangiography system. After each exposure, including injection of contrastsolution, the ascending aorta and graft were flushed with approximately100 ml 0.9% NaCl solution.

On completion of the angiographic assessment, the aorto-coronary graftwas again flushed with approximately 100 ml 0.9% NaCl solution.

In the case of an occluded aorto-coronary graft, it was not possible toestablish flow of the contrast solution through the graft. Depending onthe position of the occlusion, filling of the graft with contrastsolution could sometimes be observed angiographically. The subsequentflushing procedure was adjusted as required for occluded grafts.Angiographic recordings for each heart was transferred to an electronicstorage medium (e.g CD-ROM).

Faxitron Radiographic Assessment of Stented Grafts

Tissue-fixed explants of entire hearts with stented coronary veingrafts, and stented femoral vein grafts, were provided in 70% ethanolfor Faxitron X-ray assessment of the stented vein grafts using aFaxitron MX-20 DC4. Hearts with non-stented coronary grafts andnon-stented femoral grafts did not require Faxitron assessment. AfterFaxitron imaging and assessment, hearts with untouched stented coronaryvein grafts, and stented femoral vein grafts, were studied for furtheranalysis.

General Sectioning, Photography, and Labeling Information

Stented and non-stented aorto-coronary vein grafts were excised fromtissue-fixed hearts, and stented and non-stented femoral vein graftswere removed in an en-bloc excision, as described above. All explantedvein grafts were dissected into various sections representing proximalanastomotic region, mid region, and distal anastomotic region of thegraft. Sections included cuts perpendicular to the vein graft,generating circular sections suitable for evaluation of diameters/crosssectional areas, intimal hyperplasia, and mean medial hyperplasia.Sections included cuts parallel to the vein graft, generating stripsand/or split sections. It should be noted that stent wires in thestented grafts were non-removable. For complete documentation, explantmacro photographs of the sections were taken during the dissection.Photographs were captured, processed, and stored in a database.

Preparation and Labeling/Staining Histological Specimens

Resin embedding was performed only for sections of stented vein grafts.Certain sections of stented grafts with NiTi (Nitinol) wires in placewere processed and embedded in resin, and sectioned to 6 to 8 μm thicksections. Slides were stained with haemotoxylon & eosin, Masson'sTrichrome, Verhoefs Elastin, Azan, and Movat, as required. Wax embeddingwas only performed for sections of non-stented control vein grafts. Thedesign of the stents in this study did not permit the removal of Nitinolwires from the section, such that wax embedding of the stents was notfeasible. Sections embedded in wax were sectioned at 3 μm intervals.

Scanning Electron Microscopy (SEM) and Sample Preparation

After overnight fixation (2% glutaraldehyde in PBS), selected sectionswere dehydrated through graded ethanol, critical point dried (BalzersCPD), and gold coated (using a Polaron sputter coater). Samples wereviewed on either on a JEOL microscope or a LEO microscope.Representative images were taken of the lumenal surfaces atmagnifications in the range of 15 10,000× using an Orion electroniccapturing system (for JEOL microscope) or LEO-32 image capturing system(for LEO microscope).

Image Analysis, Evaluation, and Scoring

A Leica DM RB microscope with an attached Leitz DC200 digital camera wasused to visualize and capture macro- and histology color images of thevein grafts. Image analysis were performed using Leica's Qwin 500software. Minimum and maximum lumenal diameters were measured onphotographs of selected sections, using a commercial image analysissoftware (e.g., Leica Qwin).

For wax-embedded sections, images of Movat stained slides were capturedat a magnification of 0.5×. For resin-embedded sections, images ofhaematoxylin & eosin stained slides were captured at a magnification of0.5×. Measurements on selected cross-sectional slides were based on 5×and 10× magnification, and were performed on composite images, wherebythe entire circumference of the graft was reconstructed. On average, acomposite image consisted of 10-25 single frames. Measurements onanastomotic sections utilized 1.6× mag images for the analysis ofPannus/AIH. Measurements were made on a grid of 1 mm increments,starting from the anastomosis. All measurements were undertaken byinteractive highlighting of the area of interest and detection-filteringof the color images (achieved by ‘threshholding’). All data was recordedin a database.

The following parameters were measured:

From macroscopic images, lumenal parameters including (a) crosssectional area, (b) mean diameter calculated from internalcircumference, and (c) minimum and maximum diameters measured throughcenter of gravity of the patent lumen.

From histological sections, (1) lumenal parameters (measured from 0.5×magnification images), including (a) cross sectional area, (b) meandiameter calculated from internal circumference, and (c) minimum andmaximum diameters measured through center of gravity; and (2) intimalhyperplasia (measured from 10× magnification composites), including (a)cross sectional area, and (b) thickness (maximum, minimum, and mean);and (3) media (measured from 10× magnification composites), including(a) cross sectional area, (b) thickness (maximum, minimum, and mean),and (c) differential percentage of smooth muscle cells.

From histological sections, (1) anastomotic intimal hyperplasia/pannus(measured from 1.6× magnification images), including (a) cross sectionalarea, and (b) thickness (mean); and (2) media (measured from 10×magnification composites), including (a) cross sectional area, (b)thickness (maximum, minimum, and mean), and (c) differential percentageof smooth muscle cells.

Statistical Analysis

Individual data figures represented a mean as determined using computerplanimetry. One-way analysis of variance (ANOVA) was performed on thenumerical data using commercial software to be specified at the time.Because of the potential for unique variability in response betweenindividual animals, and the restriction of performing the surgicalimplants in groups of four animals, blocking was used in the analysis onthese factors. The analysis compared and contrasted the variousmeasurements relative to the vein graft type and position along thestent. Significance levels of 0.05 or less were accepted as beingstatistically significant.

Graft Patency

Explant patency for a total of 16 baboons with stented grafts and 13baboons with non-stented grafts is summarized in Table VIII belowPatency was established by a combination of methods including palpationassessment through the skin before incision (femoral grafts only),perfusion during in-situ perfusion fixation, angiographic assessment onexplanted hearts (CABG grafts only), and patency assessment during graftdissection for histological analysis after explanation. TABLE Effect ofGraft Position, Vein Source, Stenting, and Glue on Graft Patency TotalNo. Graft Vein No. Grafts Patency Position Source Stenting Gluing GraftsPatent Rate (%) CABG- Saph Non-stented No glue 5 4 80 LAD Femoral SaphNon-stented No glue 5 4 80 Femoral Femoral Non-stented No glue 5 4 80CABG- Saph Non-stented Glue 8 7 88 LAD Femoral Saph Non-stented Glue 8 675 Femoral Femoral Non-stented Glue 8 4 50 CABG- Saph Stented Glue 16 1488 LAD Femoral Saph Stented No glue 8 3 38 Femoral Femoral Stented Noglue 8 3 38 Femoral Saph Stented Glue 8 6 75 Femoral Femoral StentedGlue 8 7 88

Histology and Image Analysis to Determine Effects on Vein Structure

Macrophotography images of dissected explants showed that non-stentedCABG (e.g., saphenous vein without a stent) had a small lumen. Incontrast, stented CABG showed larger lumens and thin walls.

Cross-sectional images of non-stented CABG showed some remnant vesselspassing through fibrotic tissues in the vein walls. Cross-sectionalimages of stented CABG showed large lumens with little wall thickening.Holes caused by removal of stent wires prior to section preparation werealso visible.

When various parameters were measured in explanted CABG containingsaphenous vein, it was found that the non-stented CABG with saphenousvein and without glue (fibrin) had the greatest intimal hyperplasia (IH)thickness, the CABG with saphenous vein and with glue had the nextlargest IH thickness, and the stented CABG with saphenous vein and gluehad the smallest IH thickness, i.e., the least wall thickening. Inexplanted CABG, media thickness and adventitia thickness showed the samepattern. Mean lumenal area and lumenal diameter showed a pattern similarto that expected from inspection of the cross-sectional images, i.e.,non-stented CABG with saphenous vein and no glue had the smallest lumen,while the stented CABG with glue had a larger lumen. All CABG had levelsof endothelial cell coverage between 80 to 100%.

In explanted femoral grafts, non-stented femoral and saphenous veins hadthe highest mean IH thickness and mean media thickness, while stentedfemoral grafts had thinner IH and media (lower thickness). Meanadventitia thickness did not vary as much among femoral grafts that werestented or non-stented, or containing femoral or saphenous veins.Patterns of mean lumenal area and lumenal diameter were difficult tointerpret.

While the embodiments of the invention described herein are presentlypreferred, various modifications and improvements can be made withoutdeparting from the spirit and scope of the invention. The scope of theinvention is indicated by the appended claims, and all changes that fallwithin the meaning and range of equivalents are intended to be embracedtherein.

1. A flexible, resilient, generally tubular external support withinwhich may be supported a vein segment to form a venous graft mimickingthe compliance properties of a healthy artery, the tubular support beingcapable of resilient radial expansion in a manner providing compliancein the range of 3 to 30%/100 mm Hg.
 2. The tubular external support ofclaim 1, wherein said support comprises a knit wire mesh.
 3. The tubularexternal support of claim 2, wherein the knit wire mesh comprising loopsthat alternate in size circumferentially of the support
 4. The tubularexternal support of claim 1, wherein said support comprises a braidedwire mesh including an element for controlling the degree of axialextension of the support.
 5. The tubular external support of claim 4,wherein said element comprises a thread attached to and carriedinternally of the support.
 6. The tubular external support of claim 1,including a vein segment carried within the support, the vein segmenthaving an ablumenal surface in contact with, and supported by, saidtubular support.
 7. A venous graft for replacement of a section of anartery, the graft comprising a flexible, resilient, generally tubularexternal support and a vein segment carried within and having anablumenal surface in contact with and supported by the tubular support,the venous graft being capable of resilient radial expansion in a mannermimicking the compliance properties of an artery.
 8. The venous graft ofclaim 7, wherein said tubular support is capable of said resilientradial expansion without significant axial dimensional changes.
 9. Thevenous graft of claim 7, wherein the compliance of the graft ranges from3 to 30%/100 mm Hg.
 10. The venous graft of claim 7, wherein theexternal support comprises a generally tubular fiber mesh capable ofexpanding in diameter through resilient movement of wires of the mesh toaccommodate radial expansion of the vein segment supported in itsufficient to provide the graft with said compliance.
 11. The venousgraft of claim 7, wherein said external support comprises a knit,tubular mesh capable of expanding radially to accommodate radialexpansion of the vein segment supported in it within said compliancerange.
 12. The venous graft of claim 7, wherein said external supportcomprises a braided fiber mesh so configured as to exhibit radialexpansion in said compliance range without significant reduction in theaxial length of the support.
 13. The venous graft of claim 10, whereinsaid mesh is made of metal wire.
 14. The venous graft of claim 13,wherein said metal wire is of stainless steel or of a cobalt chromealloy.
 15. The venous graft of claim 13, wherein said metal wire is ashape memory alloy.
 16. The venous graft of claim 10, wherein said meshis polymeric.
 17. The venous graft of claim 7, wherein the ablumenalsurface of the vein segment is bonded to said tubular support.
 18. Thevenous graft of claim 7, wherein the vein segment and the tubularsupport are axially coextensive.
 19. The venous graft of claim 11,wherein said knitted tubular fiber mesh is made from crimped fibers. 20.The venous graft of claim 10, wherein said tubular fiber mesh is madefrom fibers that are crimped after having been formed into said tubularmesh.
 21. A venous graft for replacement of a section of an artery, thegraft comprising a flexible, resilient, knit wire mesh configured as agenerally tubular external support and a vein segment carried within andhaving an ablumenal surface in contact with and supported by the tubularsupport, the support having resilient radial expansion and contractioncharacteristics that provide the venous graft with compliance propertiesmimicking those of an artery.
 22. The venous graft of claim 21, whereinthe compliance of the venous graft ranges from 3 to 30%/100 mm Hg.
 23. Amethod of producing a venous graft for use in replacing a section of anartery, comprising providing a segment of a vein and sheathing thesegment in a generally tubular support in supportive contact with theablumenal surface of the vein segment, the support being sufficientlyflexible and radially resilient as to provide the graft with complianceproperties mimicking the compliance properties of an artery.
 24. Themethod of claim 23, further comprising cutting said venous graft from alonger section of vein sheathed in said tubular support, the ends of thevein and tubular support of the venous graft being coextensive.
 25. Themethod of claim 23, further comprising supporting said generally tubularsupport upon an exterior surface of an applicator having an internalpassage within which is positioned the vein segment, and removing theapplicator to permit the tubular support to come into supportive contactwith the ablumenal surface of the vein segment.
 26. The method of claim23, further comprising supporting said generally tubular support upon anexterior surface of an applicator having an internal passage, and, whilepassing the vein segment from within the applicator passage, drawing thetubular support onto the surface of the vein segment.
 27. The method ofclaim 23, wherein said generally tubular support is chosen to besufficiently radially resilient as to provide the venous graft with acompliance ranging from 3 to 30%/100 mm Hg.
 28. The method of claim 25,wherein said tubular support is axially resilient, the method includingthe step of controlling axial dimension changes of the tubular supportas the support comes into supportive contact with the vein segment toprovide the venous graft with said compliance properties.
 29. The methodof claim 28, further comprising including the step of providing thetubular support with an axially extending, relatively inextensibleelement that restrains the tubular support from unwanted axialextension.
 30. The method of claim 29, wherein said inextensible elementcomprises a flexible fiber positioned within the tubular support andbetween the tubular support and the ablumenal surface of the veinsegment.
 31. The method of claim 29, wherein the flexible fiber is awire joined to the interior of the tubular support.
 32. The method ofclaim 23, further comprising bonding the ablumenal surface of the veinsegment to the tubular support.
 33. The method of claim 23, furthercomprising forming said tubular support from a knit wire mesh, whereinsaid mesh is formed with loops that alternate in size circumferentiallyof the support.
 34. A method of producing a blood vessel graftcomprising providing a segment of a blood vessel and sheathing thesegment in a generally tubular support in supportive contact with theablumenal surface of the vessel segment, the support being sufficientlyflexible and radially resilient as to contribute to the complianceproperties of the graft.
 35. The method of claim 34, further comprisingsupporting the generally tubular support on the exterior surface of anintroducer tube, sliding the introducer tube over the vessel segment,and removing the introducer tube to enable the tubular support tocontact the ablumenal surface of the vessel.
 36. The method of claim 34,further comprising affixing a cord to one end of the vessel segment, andmaintaining tension on the vessel segment during sheathing of the vesselsegment within the tubular support.
 37. The method of claim 36, whereinthe cord comprises a plug frictionally received within the introducertube, the method including withdrawing the introducer tube from thevessel segment and tubular support, friction between the tube and plugmaintaining the vessel in tension during such withdrawal.
 38. A bloodvessel graft comprising a flexible, mesh configured as a generallytubular external support and a blood vessel segment carried within andhaving an ablumenal surface in contact with and supported by the tubularsupport, the support having resilient radial expansion and contractioncharacteristics contributing to the compliance properties of the graft.39. The graft of claim 38, further comprising an adhesive adhering thetubular support to the ablumenal surface of the vessel to resistcollapse of the vessel within the support.
 40. The graft of claim 38,further comprising an adhesive tape positioned around a portion of thetubular support, the tape including an adhesive that extends through thefiber mesh support and into adhesive contact with the ablumenal surfaceof the vessel.
 41. The graft of claim 39, wherein said adhesive at leastpartially embeds said fiber mesh and adhesively contacts the ablumenalsurface of the vessel.
 42. The graft of claim 39, wherein said adhesiveis formed as a coating on the fibers of the fiber mesh.
 43. The graft ofclaim 38, wherein said vessel segment is of porcine or bovine origin.44. A method of attachment of the graft of claim 40 to another bloodvessel, the method comprising cutting said portion of the graft bearingsaid adhesive tape to conform to the wall of the other blood vessel, theadhesive tape maintaining the cut end of the graft open and preventingfraying of the cut end.
 45. The method of claim 44, wherein the graft iscut on the bias.
 46. A method of applying an adhesive tape to theexterior surface of a vessel graft that includes a vessel segmentsupported within a flexible, elastic fiber mesh tubular support, themethod comprising compressing the graft between lengths of adhesive tapehaving an adhesive that penetrates the mesh to adhesively contact theablumenal surface of the vessel, and allowing the graft to resume itstubular configuration.
 47. The method of claim 46, further comprisingpositioning the tape lengths in a deformable clip, inserting a segmentof the stent in the clip between the tape lengths, deforming the clip tocompress and flatten the stent, and removing the clip.
 48. A graft forreplacement of a section of an artery, the graft comprising a flexible,resilient, knit wire mesh configured as a generally tubular externalsupport and a blood vessel segment carried within and having anablumenal surface in contact with and supported by the tubular support,the support having resilient radial expansion and contractioncharacteristics that provide the graft with compliance propertiesmimicking those of an artery.
 49. A method for selecting a stent for avenous graft, the method comprising: measuring a minimum diameter and amaximum diameter of a vein; selecting a maximum amount of downsizing forthe vein and a minimum amount of downsizing for the vein; calculating arange of diameters of stents that provide the an amount of downsizingbetween the selected maximum amount and the selected minimum amount;selecting a single stent having a diameter that falls within thecalculated range.
 50. The method according to claim 49, furthercomprising calculating a degree of downsizing for smoothening the vein.51. The method according to claim 50, wherein said smoothening comprisesaltering an outside diameter of at least one part of the vein to besubstantially the same as another part of the vein.
 52. The methodaccording to claim 49, wherein the single stent is the smallest possiblestent within the range, resulting in maximum downsizing.
 53. The methodaccording to claim 49, wherein the single stent is the largest possiblestent within the range, resulting in minimum downsizing.
 54. The methodaccording to claim 49, wherein the stent has a diameter of between about2.7 mm and about 4.0 mm.
 55. The method according to claim 53, whereinthe stent has a diameter of between about 3.0 mm and 4.0 mm.
 56. Themethod according to claim 49, wherein the maximum amount of downsizingfor the vein is the degree of downsizing for substantially smootheningthe vein.
 57. The method according to claim 56, wherein saidsubstantially smoothening the vein includes smoothening irregularitiesin the vein.
 58. A stent comprising: a generally tubular memberconstructed and arranged to receive a harvested vein, the generallytubular member being compliant so as to contract and expand with thevein, the tubular member having an inner diameter between about 2.7 mmand about 4.0 mm.
 59. The stent according to claim 58, wherein the innerdiameter is between about 3.0 mm and about 4.0 mm.
 60. The stentaccording to claim 58, wherein the generally tubular member comprises aknitted structure.
 61. The stent according to claim 60, wherein theknitted structure comprises a metal wire.
 62. The stent according toclaim 60, wherein the knitted structure comprises a polymeric material.63. The stent according to claim 62, wherein the polymeric materialcomprises an elastomeric polymer.
 64. The stent according to claim 60,wherein the knitted structure is configured to be shrinkable afterreceiving the harvested vein.
 65. The stent according to claim 58,wherein the generally tubular member comprises a plurality of connectedrings.
 66. The stent according to claim 65, wherein the rings areinterconnected.
 67. The stent according to claim 66, wherein the ringsare connected on outside surfaces of the rings.
 68. A stent deliverydevice comprising: a stent constructed and arranged to receive a veinsegment, the stent being compliant so as to contract and expand with thevein; and a delivery tube comprising a coating configured to impart slipproperties to the tube to reduce traumatic introduction of the stent toan outside surface of the vein.
 69. A method of stabilizing a veinsegment in a venous graft, the method comprising: providing the veinsegment; disposing the vein segment within a lumen of a stentconstructed and arranged to contract and expand with the vein segment,the stent having an inner diameter between about 2.7 mm and about 4.0mm.
 70. The method according to claim 69, wherein the stent comprises aknitted structure.
 71. The method according to claim 70, wherein theknitted structure comprises a metal wire.
 72. The method according toclaim 69, wherein the knitted structure comprises a polymeric material.73. The method according to claim 69, further comprising shrinking thestent after said disposing the vein segment within the lumen of thestent to smoothen irregularities in the vein segment.
 74. A method forreplacing a section of an artery in a patient with a venous graftcapable of resilient radial expansion in a manner mimicking thecompliance properties of a healthy artery in a patient comprising:restricting blood flow through said section of artery to be replaced;excising said section of said artery to be replaced, leaving a firstavailable artery end and a second available artery end in the patient;providing a vein segment having a first vein segment end and a secondvein segment end; joining said first vein segment end to said firstavailable artery end; providing a flexible, resilient, generally tubularexternal support stent capable of resilient radial expansion in a mannerproviding compliance in the range of 3 to 30%/100 mm Hg; sheathing saidvein segment with said stent by introducing said second vein segment endinto said stent and sliding said stent over the ablumenal surface ofsaid vein segment until substantially all of said vein segment iscarried within said stent; and joining the second vein segment end tothe second available artery end provide said venous graft, wherein saidvenous graft is capable of resilient radial expansion in a mannermimicking the compliance properties of a healthy artery when blood flowis restored to said artery.
 75. The method of claim 74, wherein saidstent is capable of said resilient radial expansion without significantaxial dimensional changes.
 76. The method of claim 74, wherein saidstent comprises a generally tubular fiber mesh capable of expanding indiameter through resilient movement of fibers of the mesh to accommodateradial expansion of the vein segment supported in it sufficient toprovide the venous graft with said compliance.
 77. The method of claim76, wherein said stent comprises a knit, tubular mesh capable ofexpanding radially to accommodate radial expansion of the vein segmentsupported in it, within said compliance range.
 78. The method of claim76, wherein said stent comprises a braided fiber mesh so configured asto exhibit radial expansion in said compliance range without significantreduction in the axial length of the stent.
 79. The method of claim 76,wherein said fiber mesh is made of metal wire.
 80. The method of claim79, wherein said metal wire is a shape memory alloy.
 81. The method ofclaim 76, wherein said fiber mesh is polymeric.
 82. The method of claim74, wherein said ablumenal surface of said vein segment is bonded tosaid stent.
 83. A method for reducing intimal hyperplasia in animplanted vein segment following replacement of a section of an arterywith a implantation of a venous graft comprising a providing venousgraft comprising a flexible, resilient, generally tubular externalsupport stent and said vein segment carried within said stent, said veinsegment having an ablumenal surface in contact with and supported bysaid stent, wherein said venous graft is capable of resilient radialexpansion in a manner mimicking the compliance properties of a healthyartery when blood flows through the venous graft under physiologicalconditions.
 84. The method of claim 83, said stent support being capableof resilient radial expansion in a manner providing compliance in therange of 3 to 30%/100 mm Hg.
 85. The method of claim 84, wherein saidstent is capable of said resilient radial expansion without significantaxial dimensional changes.
 86. The method of claim 84, wherein saidstent comprises a generally tubular fiber mesh capable of expanding indiameter through resilient movement of fibers of the mesh to accommodateradial expansion of the vein segment supported in it sufficient toprovide the venous graft with said compliance range.
 87. The method ofclaim 86, wherein said stent comprises a knit, tubular mesh capable ofexpanding radially to accommodate radial expansion of the vein segmentsupported in it, within said compliance range.
 88. The method of claim86, wherein said stent comprises a braided fiber mesh so configured asto exhibit radial expansion in said compliance range without significantreduction in the axial length of the stent.
 89. The method of claim 88,wherein the lumen diameter of said implanted vein segment does notincrease significantly over time.
 90. The method of claim 86, whereinsaid fiber mesh is made of metal wire.
 91. The method of claim 90,wherein said metal wire is a shape memory alloy.
 92. The method of claim86, wherein said fiber mesh is polymeric.
 93. The method according toclaim 83, further comprising shrinking the stent after said disposingthe vein segment within the lumen of the stent to smoothenirregularities in the vein segment.
 94. The method of claim 83, furthercomprising selecting a stent for the venous graft, the method furthercomprising: measuring a minimum diameter and a maximum diameter of avein from which said vein segment will harvested; selecting a maximumamount of downsizing for said vein and a minimum amount downsizing forsaid vein; calculating a range of diameters of stents that provide theamount of downsizing between said selected maximum amount and theselected minimum amount; selecting a single stent having a diameter thatfalls within said calculated range.
 95. The method according to claim94, wherein the stent has a diameter of between about 2.7 mm and about4.0 mm.
 96. The method according to claim 95, wherein the stent has adiameter of between about 3.0 mm and 4.0 mm.